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/pages/NotFound.tsx

@@ -0,0 +1,49 @@
+import { Button } from "@/components/ui/button";
+import { Card, CardContent } from "@/components/ui/card";
+import { AlertCircle, Home } from "lucide-react";
+import { useLocation } from "wouter";
+
+export default function NotFound() {
+  const [, setLocation] = useLocation();
+
+  const handleGoHome = () => {
+    setLocation("/");
+  };
+
+  return (
+    <div className="min-h-screen w-full flex items-center justify-center bg-gradient-to-br from-slate-50 to-slate-100">
+      <Card className="w-full max-w-lg mx-4 shadow-lg border-0 bg-white/80 backdrop-blur-sm">
+        <CardContent className="pt-8 pb-8 text-center">
+          <div className="flex justify-center mb-6">
+            <div className="relative">
+              <div className="absolute inset-0 bg-red-100 rounded-full animate-pulse" />
+              <AlertCircle className="relative h-16 w-16 text-red-500" />
+            </div>
+          </div>
+
+          <h1 className="text-4xl font-bold text-slate-900 mb-2">404</h1>
+
+          <h2 className="text-xl font-semibold text-slate-700 mb-4">
+            Page Not Found
+          </h2>
+
+          <p className="text-slate-600 mb-8 leading-relaxed">
+            Sorry, the page you are looking for doesn't exist.
+            <br />
+            It may have been moved or deleted.
+          </p>
+
+          <div className="flex flex-col sm:flex-row gap-3 justify-center">
+            <Button
+              onClick={handleGoHome}
+              className="bg-blue-600 hover:bg-blue-700 text-white px-6 py-2.5 rounded-lg transition-all duration-200 shadow-md hover:shadow-lg"
+            >
+              <Home className="w-4 h-4 mr-2" />
+              Go Home
+            </Button>
+          </div>
+        </CardContent>
+      </Card>
+    </div>
+  );
+}