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/usePersistFn.ts

@@ -0,0 +1,20 @@
+import { useRef } from "react";
+
+type noop = (...args: any[]) => any;
+
+/**
+ * usePersistFn instead of useCallback to reduce cognitive load
+ */
+export function usePersistFn<T extends noop>(fn: T) {
+  const fnRef = useRef<T>(fn);
+  fnRef.current = fn;
+
+  const persistFn = useRef<T>(null);
+  if (!persistFn.current) {
+    persistFn.current = function (this: unknown, ...args) {
+      return fnRef.current!.apply(this, args);
+    } as T;
+  }
+
+  return persistFn.current!;
+}