feat: watch another fight, rename nav — FIGHT! + WATCH, fix stale SW assets

- Add "WATCH ANOTHER FIGHT" button on post-fight overlay (loads random fight)
- Rename "JOIN A BOUT" → "FIGHT!" in nav, join page, register page
- Rename "ARENA" → "WATCH" in nav
- Server: return 404 for missing asset files instead of index.html (fixes "Unexpected token '<'")
- PWA: cleanupOutdatedCaches, skipWaiting, clientsClaim for instant SW updates
- Serve PWA files (sw.js, workbox, icons) with correct cache headers

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

frontend/src/pages/JoinBoutPage.vue

@@ -255,7 +255,7 @@ function handleSignOut() {
       <template v-if="step === 'login'">
         <div class="text-center mb-8">
           <h2 class="font-display font-black text-4xl tracking-wider text-neon-pink glow-pink mb-3">
-            JOIN A BOUT
+            FIGHT!
           </h2>
           <p class="font-mono text-text-muted text-xs">
             Sign in with Nostr to fight.