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/app.js

@@ -109,9 +109,46 @@ function setupKeyboardControls() {
           network.sendHardDrop();
           e.preventDefault();
           break;
+        case 'c':
+        case 'C':
+          network.sendHold();
+          e.preventDefault();
+          break;
       }
     }
   });
+
+  // Touch controls for mobile
+  const btnLeft = document.getElementById('btn-left');
+  const btnRight = document.getElementById('btn-right');
+  const btnDown = document.getElementById('btn-down');
+  const btnRotate = document.getElementById('btn-rotate');
+  const btnDrop = document.getElementById('btn-drop');
+  const btnHold = document.getElementById('btn-hold');
+
+  const handleTouch = (e, action) => {
+    e.preventDefault();
+    e.stopPropagation();
+    if (ui.screens.game.classList.contains('active')) {
+      action();
+    }
+  };
+
+  // Use pointerdown for better touch response
+  btnLeft.addEventListener('pointerdown', (e) => handleTouch(e, () => network.sendMove('left')));
+  btnRight.addEventListener('pointerdown', (e) => handleTouch(e, () => network.sendMove('right')));
+  btnDown.addEventListener('pointerdown', (e) => handleTouch(e, () => network.sendDrop()));
+  btnRotate.addEventListener('pointerdown', (e) => handleTouch(e, () => network.sendRotate()));
+  btnDrop.addEventListener('pointerdown', (e) => handleTouch(e, () => network.sendHardDrop()));
+  btnHold.addEventListener('pointerdown', (e) => handleTouch(e, () => network.sendHold()));
+
+  // Prevent double-tap zoom
+  btnLeft.addEventListener('touchstart', (e) => e.preventDefault());
+  btnRight.addEventListener('touchstart', (e) => e.preventDefault());
+  btnDown.addEventListener('touchstart', (e) => e.preventDefault());
+  btnRotate.addEventListener('touchstart', (e) => e.preventDefault());
+  btnDrop.addEventListener('touchstart', (e) => e.preventDefault());
+  btnHold.addEventListener('touchstart', (e) => e.preventDefault());
 }
 
 function updateBattleGridLayout(playerCount) {