feat(frontend): add Front/ — Vite/React frontend with backend pipeline integration

Mirror of design_agent_front/design-agent/ for shipping alongside backend.

Vite plugin (vitePluginPhaseZApi) endpoints :
  - POST /api/run   — spawn `python -m src.phase_z2_pipeline` with overrides
  - GET  /api/sample-mdx?mdx=03/04/05 — fixed sample MDX
  - GET  /frame-preview/{n} — figma preview thumbnails
  - GET  /data/runs/{run_id}/{path} — pipeline artifacts (final.html, step*.json, ...)

Env toggle forward (보고용) :
  PHASE_Z_ALLOW_RESTRUCTURE / PHASE_Z_ALLOW_REJECT / PHASE_Z_MAX_RANK=32

Components :
  - LeftMdxPanel (03/04/05 fix list + section tree)
  - SlideCanvas (iframe + slideOverrideCss prop for inline CSS inject)
  - FramePanel (label priority + confidence sort)
  - LayoutPanel

README with mermaid diagrams covering the 5-step demo flow.
node_modules / dist / .manus-logs / .env excluded via .gitignore.

Front/client/src/const.ts

@@ -0,0 +1,17 @@
+export { COOKIE_NAME, ONE_YEAR_MS } from "@shared/const";
+
+// Generate login URL at runtime so redirect URI reflects the current origin.
+export const getLoginUrl = () => {
+  const oauthPortalUrl = import.meta.env.VITE_OAUTH_PORTAL_URL;
+  const appId = import.meta.env.VITE_APP_ID;
+  const redirectUri = `${window.location.origin}/api/oauth/callback`;
+  const state = btoa(redirectUri);
+
+  const url = new URL(`${oauthPortalUrl}/app-auth`);
+  url.searchParams.set("appId", appId);
+  url.searchParams.set("redirectUri", redirectUri);
+  url.searchParams.set("state", state);
+  url.searchParams.set("type", "signIn");
+
+  return url.toString();
+};