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/components/ui/checkbox.tsx

@@ -0,0 +1,30 @@
+import * as React from "react";
+import * as CheckboxPrimitive from "@radix-ui/react-checkbox";
+import { CheckIcon } from "lucide-react";
+
+import { cn } from "@/lib/utils";
+
+function Checkbox({
+  className,
+  ...props
+}: React.ComponentProps<typeof CheckboxPrimitive.Root>) {
+  return (
+    <CheckboxPrimitive.Root
+      data-slot="checkbox"
+      className={cn(
+        "peer border-input dark:bg-input/30 data-[state=checked]:bg-primary data-[state=checked]:text-primary-foreground dark:data-[state=checked]:bg-primary data-[state=checked]:border-primary focus-visible:border-ring focus-visible:ring-ring/50 aria-invalid:ring-destructive/20 dark:aria-invalid:ring-destructive/40 aria-invalid:border-destructive size-4 shrink-0 rounded-[4px] border shadow-xs transition-shadow outline-none focus-visible:ring-[3px] disabled:cursor-not-allowed disabled:opacity-50",
+        className
+      )}
+      {...props}
+    >
+      <CheckboxPrimitive.Indicator
+        data-slot="checkbox-indicator"
+        className="flex items-center justify-center text-current transition-none"
+      >
+        <CheckIcon className="size-3.5" />
+      </CheckboxPrimitive.Indicator>
+    </CheckboxPrimitive.Root>
+  );
+}
+
+export { Checkbox };