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/contexts/ThemeContext.tsx

@@ -0,0 +1,64 @@
+import React, { createContext, useContext, useEffect, useState } from "react";
+
+type Theme = "light" | "dark";
+
+interface ThemeContextType {
+  theme: Theme;
+  toggleTheme?: () => void;
+  switchable: boolean;
+}
+
+const ThemeContext = createContext<ThemeContextType | undefined>(undefined);
+
+interface ThemeProviderProps {
+  children: React.ReactNode;
+  defaultTheme?: Theme;
+  switchable?: boolean;
+}
+
+export function ThemeProvider({
+  children,
+  defaultTheme = "light",
+  switchable = false,
+}: ThemeProviderProps) {
+  const [theme, setTheme] = useState<Theme>(() => {
+    if (switchable) {
+      const stored = localStorage.getItem("theme");
+      return (stored as Theme) || defaultTheme;
+    }
+    return defaultTheme;
+  });
+
+  useEffect(() => {
+    const root = document.documentElement;
+    if (theme === "dark") {
+      root.classList.add("dark");
+    } else {
+      root.classList.remove("dark");
+    }
+
+    if (switchable) {
+      localStorage.setItem("theme", theme);
+    }
+  }, [theme, switchable]);
+
+  const toggleTheme = switchable
+    ? () => {
+        setTheme(prev => (prev === "light" ? "dark" : "light"));
+      }
+    : undefined;
+
+  return (
+    <ThemeContext.Provider value={{ theme, toggleTheme, switchable }}>
+      {children}
+    </ThemeContext.Provider>
+  );
+}
+
+export function useTheme() {
+  const context = useContext(ThemeContext);
+  if (!context) {
+    throw new Error("useTheme must be used within ThemeProvider");
+  }
+  return context;
+}