diff --git a/devfront/src/components/common/LanguageSelector.test.tsx b/devfront/src/components/common/LanguageSelector.test.tsx
new file mode 100644
index 00000000..302dd23c
--- /dev/null
+++ b/devfront/src/components/common/LanguageSelector.test.tsx
@@ -0,0 +1,77 @@
+import { act } from "react";
+import { createRoot, type Root } from "react-dom/client";
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+
+vi.mock("../../lib/i18n", () => ({
+  t: (_key: string, fallback?: string) => fallback ?? _key,
+}));
+
+import LanguageSelector from "./LanguageSelector";
+
+const roots: Root[] = [];
+
+beforeEach(() => {
+  window.localStorage.clear();
+  window.history.replaceState({}, "", "/");
+  document.body.innerHTML = "";
+});
+
+afterEach(() => {
+  for (const root of roots.splice(0)) {
+    act(() => {
+      root.unmount();
+    });
+  }
+  vi.restoreAllMocks();
+});
+
+function renderSelector() {
+  const container = document.createElement("div");
+  document.body.appendChild(container);
+  const root = createRoot(container);
+  roots.push(root);
+
+  act(() => {
+    root.render(<LanguageSelector />);
+  });
+
+  return container;
+}
+
+describe("LanguageSelector", () => {
+  it("prefers the locale stored in localStorage", () => {
+    window.localStorage.setItem("locale", "en");
+
+    const container = renderSelector();
+    const select = container.querySelector("select") as HTMLSelectElement;
+
+    expect(select.value).toBe("en");
+  });
+
+  it("falls back to the path locale when storage is empty", () => {
+    window.history.replaceState({}, "", "/ko");
+
+    const container = renderSelector();
+    const select = container.querySelector("select") as HTMLSelectElement;
+
+    expect(select.value).toBe("ko");
+  });
+
+  it("saves the selected locale and dispatches a development event", () => {
+    vi.stubEnv("MODE", "development");
+    const dispatchEvent = vi.spyOn(window, "dispatchEvent");
+    window.history.replaceState({}, "", "/ko");
+
+    const container = renderSelector();
+    const select = container.querySelector("select") as HTMLSelectElement;
+
+    act(() => {
+      select.value = "en";
+      select.dispatchEvent(new Event("change", { bubbles: true }));
+    });
+
+    expect(window.localStorage.getItem("locale")).toBe("en");
+    expect(dispatchEvent).toHaveBeenCalled();
+    expect(select.value).toBe("en");
+  });
+});
diff --git a/devfront/src/components/ui/copy-button.test.tsx b/devfront/src/components/ui/copy-button.test.tsx
new file mode 100644
index 00000000..9ddea7e5
--- /dev/null
+++ b/devfront/src/components/ui/copy-button.test.tsx
@@ -0,0 +1,82 @@
+import { act } from "react";
+import { createRoot, type Root } from "react-dom/client";
+import { afterEach, describe, expect, it, vi } from "vitest";
+import { CopyButton } from "./copy-button";
+
+const roots: Root[] = [];
+
+afterEach(() => {
+  for (const root of roots.splice(0)) {
+    act(() => {
+      root.unmount();
+    });
+  }
+  vi.restoreAllMocks();
+  delete (navigator as Navigator & { clipboard?: unknown }).clipboard;
+  Object.defineProperty(window, "isSecureContext", {
+    value: false,
+    configurable: true,
+  });
+  document.body.innerHTML = "";
+});
+
+function renderCopyButton(value: string, onCopy = vi.fn()) {
+  const container = document.createElement("div");
+  document.body.appendChild(container);
+  const root = createRoot(container);
+  roots.push(root);
+
+  act(() => {
+    root.render(<CopyButton value={value} onCopy={onCopy} />);
+  });
+
+  return { container, onCopy };
+}
+
+describe("CopyButton", () => {
+  it("copies with the clipboard API when secure context is available", async () => {
+    const writeText = vi.fn().mockResolvedValue(undefined);
+    Object.defineProperty(navigator, "clipboard", {
+      value: { writeText },
+      configurable: true,
+    });
+    Object.defineProperty(window, "isSecureContext", {
+      value: true,
+      configurable: true,
+    });
+
+    const { container, onCopy } = renderCopyButton("client-secret");
+    const button = container.querySelector("button");
+    expect(button).not.toBeNull();
+
+    await act(async () => {
+      button?.dispatchEvent(new MouseEvent("click", { bubbles: true }));
+    });
+
+    expect(writeText).toHaveBeenCalledWith("client-secret");
+    expect(onCopy).toHaveBeenCalledTimes(1);
+  });
+
+  it("falls back to execCommand when clipboard API is unavailable", async () => {
+    const execCommand = vi.fn(() => true);
+    Object.defineProperty(document, "execCommand", {
+      value: execCommand,
+      configurable: true,
+    });
+    Object.defineProperty(window, "isSecureContext", {
+      value: false,
+      configurable: true,
+    });
+
+    const { container, onCopy } = renderCopyButton("client-secret");
+    const button = container.querySelector("button");
+    expect(button).not.toBeNull();
+
+    await act(async () => {
+      button?.dispatchEvent(new MouseEvent("click", { bubbles: true }));
+    });
+
+    expect(execCommand).toHaveBeenCalledWith("copy");
+    expect(onCopy).toHaveBeenCalledTimes(1);
+  });
+});
diff --git a/devfront/src/features/coverage/commonAudit.test.ts b/devfront/src/features/coverage/commonAudit.test.ts
new file mode 100644
index 00000000..43add445
--- /dev/null
+++ b/devfront/src/features/coverage/commonAudit.test.ts
@@ -0,0 +1,72 @@
+import { describe, expect, it } from "vitest";
+import {
+  formatAuditDateParts,
+  formatAuditValue,
+  parseAuditDetails,
+  resolveAuditAction,
+  resolveAuditActor,
+  resolveAuditTarget,
+} from "../../../../common/core/audit";
+
+describe("common audit helpers", () => {
+  it("parses audit details and falls back on invalid payloads", () => {
+    expect(parseAuditDetails()).toEqual({});
+    expect(parseAuditDetails("not-json")).toEqual({});
+    expect(parseAuditDetails('{"action":"ADD_RELATION"}')).toEqual({
+      action: "ADD_RELATION",
+    });
+  });
+
+  it("formats audit values and dates", () => {
+    const circular: Record<string, unknown> = {};
+    circular.self = circular;
+
+    expect(formatAuditValue(null)).toBe("-");
+    expect(formatAuditValue("hello")).toBe("hello");
+    expect(formatAuditValue({ a: 1 })).toBe('{"a":1}');
+    expect(formatAuditValue(circular)).toBe("[object Object]");
+
+    expect(formatAuditDateParts("")).toEqual({ date: "-", time: "-" });
+    expect(formatAuditDateParts("invalid")).toEqual({
+      date: "invalid",
+      time: "-",
+    });
+
+    const parsed = formatAuditDateParts("2026-05-27T07:43:39.000Z");
+    expect(parsed.date).toBe("2026-05-27");
+    expect(parsed.time).not.toBe("-");
+  });
+
+  it("resolves audit actor, action, and target consistently", () => {
+    expect(
+      resolveAuditActor(
+        { user_id: "actor-1" },
+        { actor_id: "actor-2" },
+      ),
+    ).toBe("actor-1");
+    expect(
+      resolveAuditActor({ user_id: "" }, { actor_id: "actor-2" }),
+    ).toBe("actor-2");
+    expect(resolveAuditActor({ user_id: "" }, {})).toBe("-");
+
+    expect(
+      resolveAuditAction(
+        { event_type: "UPDATE_CLIENT" },
+        { action: "ADD_RELATION" },
+      ),
+    ).toBe("ADD_RELATION");
+    expect(
+      resolveAuditAction(
+        { event_type: "UPDATE_CLIENT" },
+        { method: "POST", path: "/dev/clients" },
+      ),
+    ).toBe("POST /dev/clients");
+    expect(resolveAuditAction({ event_type: "UPDATE_CLIENT" }, {})).toBe(
+      "UPDATE_CLIENT",
+    );
+
+    expect(resolveAuditTarget({ target: "target-1" })).toBe("target-1");
+    expect(resolveAuditTarget({ target_id: "target-2" })).toBe("target-2");
+    expect(resolveAuditTarget({})).toBe("-");
+  });
+});
diff --git a/devfront/src/features/coverage/commonAuth.test.ts b/devfront/src/features/coverage/commonAuth.test.ts
new file mode 100644
index 00000000..5618e53d
--- /dev/null
+++ b/devfront/src/features/coverage/commonAuth.test.ts
@@ -0,0 +1,72 @@
+import { describe, expect, it } from "vitest";
+import {
+  DEFAULT_OIDC_REDIRECT_PATH,
+  DEFAULT_OIDC_SCOPE,
+  buildCommonOidcRuntimeConfig,
+  buildCommonUserManagerSettings,
+  shouldStartLoginRedirect,
+} from "../../../../common/core/auth";
+
+describe("common auth helpers", () => {
+  it("builds the runtime OIDC config with sensible defaults", () => {
+    const config = buildCommonOidcRuntimeConfig({
+      authority: "https://issuer.example.com",
+      clientId: "client-1",
+      userStore: { kind: "store" },
+    });
+
+    expect(config).toEqual({
+      authority: "https://issuer.example.com",
+      client_id: "client-1",
+      redirect_uri: `${window.location.origin}${DEFAULT_OIDC_REDIRECT_PATH}`,
+      response_type: "code",
+      scope: DEFAULT_OIDC_SCOPE,
+      post_logout_redirect_uri: window.location.origin,
+      popup_redirect_uri: `${window.location.origin}${DEFAULT_OIDC_REDIRECT_PATH}`,
+      userStore: { kind: "store" },
+      automaticSilentRenew: false,
+    });
+  });
+
+  it("copies user manager config and fills missing string fields", () => {
+    expect(
+      buildCommonUserManagerSettings({
+        authority: "https://issuer.example.com",
+      }),
+    ).toEqual({
+      authority: "https://issuer.example.com",
+      client_id: "",
+      redirect_uri: "",
+    });
+  });
+
+  it("decides when to start login redirects", () => {
+    expect(
+      shouldStartLoginRedirect({
+        pathname: "/clients",
+        isRedirecting: false,
+      }),
+    ).toBe(true);
+
+    expect(
+      shouldStartLoginRedirect({
+        pathname: "/login",
+        isRedirecting: false,
+      }),
+    ).toBe(false);
+
+    expect(
+      shouldStartLoginRedirect({
+        pathname: `${DEFAULT_OIDC_REDIRECT_PATH}/callback`,
+        isRedirecting: false,
+      }),
+    ).toBe(false);
+
+    expect(
+      shouldStartLoginRedirect({
+        pathname: "/clients",
+        isRedirecting: true,
+      }),
+    ).toBe(false);
+  });
+});
diff --git a/devfront/src/features/overview/recentClientChanges.test.ts b/devfront/src/features/overview/recentClientChanges.test.ts
new file mode 100644
index 00000000..8163f10e
--- /dev/null
+++ b/devfront/src/features/overview/recentClientChanges.test.ts
@@ -0,0 +1,201 @@
+import { beforeEach, describe, expect, it } from "vitest";
+import type { ClientSummary, DevAuditLog } from "../../lib/devApi";
+import {
+  buildRecentClientChangeDetails,
+  buildRecentClientChanges,
+  getRecentClientActionLabel,
+} from "./recentClientChanges";
+
+function makeClient(id: string, name = id): ClientSummary {
+  return {
+    id,
+    name,
+    type: "private",
+    status: "active",
+    createdAt: "2026-05-27T00:00:00.000Z",
+    redirectUris: [],
+    scopes: [],
+  };
+}
+
+function makeAuditLog(
+  eventId: string,
+  timestamp: string,
+  action: string,
+  targetId: string,
+  details: Record<string, unknown>,
+): DevAuditLog {
+  return {
+    event_id: eventId,
+    timestamp,
+    user_id: "actor-1",
+    event_type: "AUDIT",
+    status: "success",
+    ip_address: "127.0.0.1",
+    user_agent: "vitest",
+    details: JSON.stringify({
+      action,
+      target_id: targetId,
+      ...details,
+    }),
+  };
+}
+
+describe("recent client changes", () => {
+  beforeEach(() => {
+    window.localStorage.clear();
+    window.history.replaceState({}, "", "/");
+  });
+
+  function mockLocale(locale: "ko" | "en") {
+    window.localStorage.clear();
+    window.history.replaceState({}, "", `/${locale}`);
+  }
+
+  it("translates action labels and relation details by locale", () => {
+    mockLocale("en");
+
+    expect(getRecentClientActionLabel("CREATE_CLIENT")).toBe("App creation");
+    expect(getRecentClientActionLabel("UPDATE_CLIENT")).toBe("Settings changes");
+    expect(getRecentClientActionLabel("UPDATE_CLIENT_STATUS")).toBe(
+      "Status changes",
+    );
+    expect(getRecentClientActionLabel("ROTATE_SECRET")).toBe(
+      "Client secret rotation",
+    );
+    expect(getRecentClientActionLabel("ADD_RELATION")).toBe("Add Relationship");
+    expect(getRecentClientActionLabel("REMOVE_RELATION")).toBe("Remove");
+    expect(getRecentClientActionLabel("DELETE_CLIENT")).toBe("App deletion");
+    expect(getRecentClientActionLabel("OTHER_ACTION")).toBe("OTHER_ACTION");
+
+    expect(
+      buildRecentClientChangeDetails("ROTATE_SECRET", {
+        after: {},
+      }),
+    ).toEqual([{ label: "Client Secret", value: "Secret Rotated" }]);
+
+    expect(
+      buildRecentClientChangeDetails("ADD_RELATION", {
+        after: {
+          relation: "admins",
+          subject: "User:1",
+        },
+      }),
+    ).toEqual([
+      { label: "Relation", value: "admins" },
+      { label: "Subject", value: "User:1" },
+    ]);
+  });
+
+  it("builds recent client changes with sorting, filtering, and detail slicing", () => {
+    mockLocale("ko");
+
+    const clients = [makeClient("client-a", "Alpha"), makeClient("client-b", "")];
+    const auditLogs = [
+      makeAuditLog("evt-1", "2026-05-27T07:00:00.000Z", "CREATE_CLIENT", "client-a", {
+        after: { name: "Alpha", type: "private", status: "active" },
+      }),
+      makeAuditLog("evt-2", "2026-05-27T08:00:00.000Z", "UPDATE_CLIENT", "client-a", {
+        before: {
+          name: "Alpha old",
+          status: "inactive",
+          sameField: "same",
+          oldField: "old-value",
+        },
+        after: {
+          name: "Alpha new",
+          status: "active",
+          sameField: "same",
+          newField: "new-value",
+        },
+      }),
+      makeAuditLog("evt-3", "2026-05-27T09:00:00.000Z", "UPDATE_CLIENT_STATUS", "client-a", {
+        before: { status: "inactive" },
+        after: { status: "active" },
+      }),
+      makeAuditLog("evt-4", "2026-05-27T10:00:00.000Z", "ADD_RELATION", "client-b", {
+        after: {
+          relation: "audit_viewer",
+          subject: "User:89692983-f512-4d96-845d-ac6123d08b95",
+        },
+      }),
+      makeAuditLog("evt-5", "2026-05-27T11:00:00.000Z", "REMOVE_RELATION", "client-b", {
+        before: {
+          relation: "admins",
+          subject: "User:89692983-f512-4d96-845d-ac6123d08b95",
+        },
+      }),
+      makeAuditLog("evt-6", "2026-05-27T12:00:00.000Z", "ROTATE_SECRET", "client-a", {
+        after: {},
+      }),
+      makeAuditLog("evt-7", "2026-05-27T13:00:00.000Z", "DELETE_CLIENT", "client-a", {
+        before: {
+          name: "Alpha",
+          status: "inactive",
+        },
+      }),
+      makeAuditLog("evt-8", "2026-05-27T14:00:00.000Z", "UNSUPPORTED_ACTION", "client-a", {
+        after: { name: "Ignored" },
+      }),
+    ];
+
+    const changes = buildRecentClientChanges(
+      auditLogs,
+      clients,
+    );
+
+    expect(changes).toHaveLength(7);
+    expect(changes[0]).toMatchObject({
+      eventId: "evt-7",
+      clientName: "Alpha",
+      actionLabel: "앱 삭제",
+    });
+    expect(changes[1]).toMatchObject({
+      eventId: "evt-6",
+      clientName: "Alpha",
+      actionLabel: "클라이언트 시크릿 재발급",
+      detailLabels: [
+        {
+          label: "클라이언트 시크릿",
+          value: "Client Secret이 재발급되었습니다.",
+        },
+      ],
+    });
+    expect(changes[2]).toMatchObject({
+      eventId: "evt-5",
+      clientName: "client-b",
+      actionLabel: "제외",
+      detailLabels: [
+        { label: "관계", value: "admins" },
+        {
+          label: "주체",
+          value: "User:89692983-f512-4d96-845d-ac6123d08b95",
+        },
+      ],
+    });
+    expect(changes[4]).toMatchObject({
+      eventId: "evt-3",
+      actionLabel: "상태 변경",
+      clientName: "Alpha",
+      detailLabels: [{ value: "inactive → active" }],
+    });
+    expect(changes[5]).toMatchObject({
+      eventId: "evt-2",
+      actionLabel: "설정 변경",
+      detailLabels: [
+        { label: "애플리케이션", value: "Alpha old → Alpha new" },
+        { label: "상태", value: "inactive → active" },
+        { label: "oldField", value: "old-value" },
+      ],
+    });
+    expect(changes[6]).toMatchObject({
+      eventId: "evt-1",
+      actionLabel: "앱 생성",
+      detailLabels: [
+        { label: "애플리케이션", value: "Alpha" },
+        { label: "유형", value: "private" },
+        { label: "상태", value: "active" },
+      ],
+    });
+  });
+});
diff --git a/devfront/src/features/overview/recentClientChanges.ts b/devfront/src/features/overview/recentClientChanges.ts
index 6284f58d..534cb86a 100644
--- a/devfront/src/features/overview/recentClientChanges.ts
+++ b/devfront/src/features/overview/recentClientChanges.ts
@@ -59,6 +59,12 @@ export function getRecentClientActionLabel(action: string) {
 
 function getRecentClientFieldLabel(key: string) {
   switch (key) {
+    case "name":
+      return t("ui.dev.clients.table.application", "Application");
+    case "type":
+      return t("ui.dev.clients.table.type", "Type");
+    case "status":
+      return t("ui.dev.clients.table.status", "Status");
     case "relation":
       return t("ui.dev.clients.relationships.relation", "관계");
     case "subject":
@@ -84,7 +90,7 @@ export function buildRecentClientChangeDetails(
     return [
       {
         label: getRecentClientFieldLabel("client_secret"),
-        value: t("msg.dev.clients.secret_rotated", "재발급"),
+        value: t("msg.dev.clients.details.secret_rotated", "재발급"),
       },
     ];
   }