feat: fight feed page with filters

Reverse-chronological activity stream of recent fights showing winner,
KO/decision result, arena, sats, and time ago. Filter by tier and bot
name. Click any fight to watch replay. Added to NavBar as FEED link.

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

frontend/src/components/NavBar.vue

@@ -11,6 +11,7 @@ const links = [
   { to: '/join', label: 'FIGHT!' },
   { to: '/fight-card', label: 'FIGHT CARD' },
   { to: '/arena', label: 'WATCH' },
+  { to: '/feed', label: 'FEED' },
   { to: '/leaderboard', label: 'RANKINGS' },
   { to: '/docs', label: 'DOCS' },
 ]