Prevent sheet editing after game completion and add game start celebration with enhanced splash screen animations

Added read-only enforcement for finished games by checking `winner_user_id` in backend PATCH endpoint and frontend sheet interactions. Implemented GameStartCelebration component with confetti burst, animated overlay card, and pulsing rune icon. Enhanced splash screen with orbiting border animation, sparkle effects, and title rise transition. Added haptic feedback (vibration) to sheet

backend/app/routes/games.py

@@ -308,6 +308,9 @@ def patch_sheet(req: Request, game_id: str, entry_id: str, data: dict, db: Sessi
     uid = require_user(req, db)
     g = require_game_member(db, game_id, uid)
 
+    if g.winner_user_id:
+        raise HTTPException(403, "game finished; sheet is read-only")
+
     status = data.get("status")
     note_tag = data.get("note_tag")
     chip = data.get("chip")