Wire client SignalrService to the live .NET backend

- @microsoft/signalr client implementing OnlineService: REST auth, hub
  matchmaking, server-driven game state (onState), play/trump, reactions;
  delegates not-yet-server-backed features (profile/friends/shop/chat/rooms)
  to the mock. Selected via NEXT_PUBLIC_USE_SERVER=1 (NEXT_PUBLIC_SERVER_URL)
- game-store live mode: enterServerMatch + applyServerState (maps server DTO,
  hides opponent hands, tally + SFX), inputs route to the hub; no local engine
- MatchmakingScreen auto-enters the live match when the server signals ready
- Verified end-to-end via scripts/live-test.mjs (auth -> hub -> match -> state)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitignore

@@ -32,6 +32,7 @@ yarn-error.log*
 
 # env files (can opt-in for committing if needed)
 .env*
+!.env.example
 
 # vercel
 .vercel