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/vite.config.ts

@@ -0,0 +1,24 @@
+import { defineConfig } from 'vitest/config'
+import react from '@vitejs/plugin-react'
+
+// https://vitejs.dev/config/
+export default defineConfig({
+  plugins: [react()],
+
+  server: {
+    proxy: {
+      '/api': 'http://localhost:8000',
+      '/login': 'http://localhost:8000',
+      '/logout': 'http://localhost:8000',
+      '/static': 'http://localhost:8000',
+      '/docs': 'http://localhost:8000',
+      '/openapi.json': 'http://localhost:8000',
+    },
+  },
+
+  test: {
+    environment: 'jsdom',
+    globals: true,
+    setupFiles: ['./src/test-setup.ts'],
+  },
+})