fix: stale SW chunks serving HTML, PWA cache cleanup, server asset routing

- Server SPA fallback now returns 404 for missing .js/.css assets instead of index.html
- Prevents "Unexpected token '<'" errors from stale chunk URLs
- Serve PWA files (sw.js, workbox, icons) with correct cache headers
- Workbox: cleanupOutdatedCaches, skipWaiting, clientsClaim for instant SW updates
- navigateFallbackDenylist for /api/ routes
- AudioContext comment clarification

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

frontend/vite.config.ts

@@ -28,7 +28,11 @@ export default defineConfig({
       },
       workbox: {
         globPatterns: ['**/*.{js,css,html,svg,png,woff,woff2}'],
+        cleanupOutdatedCaches: true,
+        skipWaiting: true,
+        clientsClaim: true,
         navigateFallback: '/index.html',
+        navigateFallbackDenylist: [/^\/api\//],
         runtimeCaching: [
           {
             urlPattern: /^https:\/\/fonts\.googleapis\.com\/.*/i,