Add ghost piece preview showing where block will land

- Add getGhostY() method to TetrisGame class (game.js)
- Add ghostY to getState() output
- Add getGhostY() helper to server (server/index.js)
- Include ghostY in getStates() broadcast
- Add drawGhostPiece() and drawGhostCell() methods (renderer.js)
- Ghost renders as semi-transparent outline at landing position

public/js/renderer.js

@@ -98,6 +98,11 @@ class TetrisRenderer {
       this.drawBoard(ctx, gameState.board);
     }
 
+    // Draw ghost piece (landing preview)
+    if (gameState && gameState.currentPiece && !gameState.gameOver && gameState.ghostY !== null) {
+      this.drawGhostPiece(ctx, gameState.currentPiece, gameState.ghostY);
+    }
+
     // Draw current piece
     if (gameState && gameState.currentPiece && !gameState.gameOver) {
       this.drawPiece(ctx, gameState.currentPiece);
@@ -210,6 +215,34 @@ class TetrisRenderer {
     ctx.strokeRect(px, py, CELL_SIZE, CELL_SIZE);
   }
 
+  drawGhostPiece(ctx, piece, ghostY) {
+    for (let row = 0; row < piece.shape.length; row++) {
+      for (let col = 0; col < piece.shape[row].length; col++) {
+        if (piece.shape[row][col]) {
+          const x = piece.x + col;
+          const y = ghostY + row;
+          if (y >= 0) {
+            this.drawGhostCell(ctx, x, y, piece.color);
+          }
+        }
+      }
+    }
+  }
+
+  drawGhostCell(ctx, x, y, color) {
+    const px = x * CELL_SIZE;
+    const py = y * CELL_SIZE;
+
+    // Semi-transparent fill with outlined border effect
+    ctx.strokeStyle = color + '80'; // 50% opacity border
+    ctx.lineWidth = 2;
+    ctx.strokeRect(px + 2, py + 2, CELL_SIZE - 4, CELL_SIZE - 4);
+
+    // Light fill inside
+    ctx.fillStyle = color + '20'; // 12.5% opacity
+    ctx.fillRect(px + 3, py + 3, CELL_SIZE - 6, CELL_SIZE - 6);
+  }
+
   drawNextPiece(ctx, piece) {
     // Clear
     ctx.fillStyle = '#000';