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/components/NavBar.vue

@@ -8,8 +8,8 @@ const { bot, isLoggedIn } = useNostr()
 const isMenuOpen = ref(false)
 
 const links = [
-  { to: '/join', label: 'JOIN A BOUT' },
-  { to: '/arena', label: 'ARENA' },
+  { to: '/join', label: 'FIGHT!' },
+  { to: '/arena', label: 'WATCH' },
   { to: '/leaderboard', label: 'RANKINGS' },
 ]
 </script>