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/hooks/useMobile.tsx

@@ -0,0 +1,21 @@
+import * as React from "react";
+
+const MOBILE_BREAKPOINT = 768;
+
+export function useIsMobile() {
+  const [isMobile, setIsMobile] = React.useState<boolean | undefined>(
+    undefined
+  );
+
+  React.useEffect(() => {
+    const mql = window.matchMedia(`(max-width: ${MOBILE_BREAKPOINT - 1}px)`);
+    const onChange = () => {
+      setIsMobile(window.innerWidth < MOBILE_BREAKPOINT);
+    };
+    mql.addEventListener("change", onChange);
+    setIsMobile(window.innerWidth < MOBILE_BREAKPOINT);
+    return () => mql.removeEventListener("change", onChange);
+  }, []);
+
+  return !!isMobile;
+}