M2-T06: scaffold React SPA frontend with typed OpenAPI client

- Vite + React 18 + TypeScript + Mantine + TanStack Query + react-router-dom
- typed client: openapi-typescript -> src/api/schema.d.ts (committed), openapi-fetch
- fetch wrapper middleware: cookies, X-CSRF-Token on writes, 401 -> /login,
  non-401 errors carry parsed JSON body
- SessionProvider/useSession (GET /api/session), ProtectedRoute skeleton
- app shell (Mantine + router) with placeholder login/home/config pages + gear nav
- dev proxy to FastAPI; vitest smoke test; frontend README
- npm scripts: dev/build/preview/lint/typecheck/test/codegen

frontend/src/main.tsx

@@ -0,0 +1,18 @@
+/**
+ * Entry point — mounts the React app into #root.
+ */
+
+import { StrictMode } from 'react'
+import { createRoot } from 'react-dom/client'
+import App from './App'
+
+const rootElement = document.getElementById('root')
+if (!rootElement) {
+  throw new Error('Root element #root not found in document')
+}
+
+createRoot(rootElement).render(
+  <StrictMode>
+    <App />
+  </StrictMode>,
+)