Add hold piece feature

- Added holdPiece and canHold state to TetrisGame class
- Implemented hold() method to swap current piece with held piece
- Added player-hold socket event on server
- Added HOLD preview canvas showing held piece (grayed when unavailable)
- Added C key keyboard shortcut and touch button for hold
- Fixed canHold reset on piece spawn for proper swap functionality

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

public/js/network.js

@@ -98,6 +98,11 @@ class NetworkManager {
     this.socket.emit('player-drop', { playerId: this.currentPlayerId, hard: true });
   }
 
+  sendHold() {
+    if (!this.socket || !this.currentPlayerId) return;
+    this.socket.emit('player-hold', { playerId: this.currentPlayerId });
+  }
+
   updatePlayers(players) {
     this.players = {};
     players.forEach(p => {