feat(frontend): scaffold Vite+React+TS+Tailwind/ShadCn + Vitest

TDD: App-renders-ThothII test RED then GREEN. ShadCn v4 (base-nova style)
init + button/checkbox/radio-group/textarea/card/dialog/badge/sonner added.
tailwind.config.ts extended with oklch CSS-variable color mapping required
by ShadCn v4 @apply directives. npm test passes; npm run build clean.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/main.tsx

@@ -0,0 +1,10 @@
+import { StrictMode } from "react";
+import { createRoot } from "react-dom/client";
+import { App } from "./App";
+import "./index.css";
+
+createRoot(document.getElementById("root")!).render(
+  <StrictMode>
+    <App />
+  </StrictMode>
+);