feat: tournament bracket UI

TournamentListPage with status filters, TournamentPage with CSS grid
bracket display, lobby view for open tournaments, champion banner,
live polling for active tournaments. Routes at /tournaments and
/tournament/:id. NavBar link added.

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

frontend/src/components/NavBar.vue

@@ -12,6 +12,7 @@ const links = [
   { to: '/fight-card', label: 'FIGHT CARD' },
   { to: '/arena', label: 'WATCH' },
   { to: '/feed', label: 'FEED' },
+  { to: '/tournaments', label: 'TOURNEYS' },
   { to: '/leaderboard', label: 'RANKINGS' },
   { to: '/docs', label: 'DOCS' },
 ]