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>