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/src/game/sounds.ts

@@ -5,6 +5,9 @@ let sfxGain: GainNode | null = null
 let musicPlaying = false
 let musicTimeout: number | null = null
 
+// Lazily create AudioContext + gain nodes on first use.
+// Chrome may warn about autoplay policy but the context will resume once ensureAudioContext()
+// is called from a user gesture. All SFX calls before that are queued/silent.
 function getCtx(): AudioContext {
   if (!ctx) {
     ctx = new AudioContext()