Add game cancellation feature with cascade deletion and host-only access control

Implemented DELETE `/games/{game_id}` endpoint allowing hosts to cancel games with cascade deletion of sheet states, chips, and memberships. Added `cancelGame` handler in App.jsx to delete game, refresh game list, reset state, and show confirmation snack. Added cancel button to GamePickerCard lobby section (pre-start) and started section (non-finished games) with bilingual confirmation dialog, loading state, and error handling.

backend/app/routes/games.py

@@ -205,6 +205,22 @@ def start_game(req: Request, game_id: str, db: Session = Depends(get_db)):
     return {"ok": True, "started": True}
 
 
+@router.delete("/{game_id}")
+def cancel_game(req: Request, game_id: str, db: Session = Depends(get_db)):
+    uid = require_user(req, db)
+    g = require_game_member(db, game_id, uid)
+    if g.host_user_id != uid:
+        raise HTTPException(403, "only host can cancel the game")
+
+    # Remove dependent game state before deleting the game itself.
+    db.query(SheetState).filter(SheetState.game_id == game_id).delete(synchronize_session=False)
+    db.query(GameChip).filter(GameChip.game_id == game_id).delete(synchronize_session=False)
+    db.query(GameMember).filter(GameMember.game_id == game_id).delete(synchronize_session=False)
+    db.delete(g)
+    db.commit()
+    return {"ok": True, "cancelled": True}
+
+
 @router.get("/{game_id}/chips")
 def list_game_chips(req: Request, game_id: str, db: Session = Depends(get_db)):
     uid = require_user(req, db)