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/game.js

@@ -280,6 +280,16 @@ class TetrisGame {
     this.dropInterval = Math.max(100, 1000 - (this.level - 1) * 100);
   }
 
+  getGhostY() {
+    if (!this.currentPiece) return null;
+
+    let ghostY = this.currentPiece.y;
+    while (this.isValidPosition(this.currentPiece.x, ghostY + 1, this.currentPiece.shape)) {
+      ghostY++;
+    }
+    return ghostY;
+  }
+
   receiveGarbage(rows) {
     if (this.gameOver) return;
 
@@ -328,10 +338,12 @@ class TetrisGame {
   }
 
   getState() {
+    const ghostY = this.currentPiece ? this.getGhostY() : null;
     return {
       playerId: this.playerId,
       board: JSON.parse(JSON.stringify(this.board)),
       currentPiece: this.currentPiece ? JSON.parse(JSON.stringify(this.currentPiece)) : null,
+      ghostY: ghostY,
       nextPiece: this.nextPiece ? JSON.parse(JSON.stringify(this.nextPiece)) : null,
       holdPiece: this.holdPiece ? JSON.parse(JSON.stringify(this.holdPiece)) : null,
       canHold: this.canHold,