feat: v2 React frontend is now primary at /

- v1 vanilla JS frontend moved to /classic (static/classic/)
- v2 React app now serves at / (root)
- Vite base changed from /v2/ to /
- Assets at /assets/, service worker at /sw.js
- /classic still works — all v1 files preserved with relative paths
- /v2 still works as before (build output unchanged)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/main.tsx

@@ -10,5 +10,5 @@ createRoot(document.getElementById('root')!).render(
 
 // Register service worker for asset caching
 if ('serviceWorker' in navigator) {
-  navigator.serviceWorker.register('/v2/sw.js').catch(() => {});
+  navigator.serviceWorker.register('/sw.js').catch(() => {});
 }

frontend/vite.config.ts

@@ -3,9 +3,9 @@ import react from '@vitejs/plugin-react'
 
 export default defineConfig({
   plugins: [react()],
-  base: '/v2/',
+  base: '/',
   build: {
-    outDir: '../static/v2',
+    outDir: '../static/v2',  // still build to v2/ first, then we copy
     emptyOutDir: true,
     chunkSizeWarningLimit: 300,
     rollupOptions: {