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

server/index.js

@@ -338,6 +338,16 @@ function addGarbageToPlayer(player) {
   // Elimination is handled by spawnPiece when a new piece can't fit
 }
 
+function getGhostY(piece, board) {
+  if (!piece) return null;
+
+  let ghostY = piece.y;
+  while (isValidPosition(piece, piece.x, ghostY + 1, board)) {
+    ghostY++;
+  }
+  return ghostY;
+}
+
 function startGame() {
   lobby.gameStarted = true;
 
@@ -391,6 +401,7 @@ function getStates() {
     playerId: p.id,
     board: JSON.parse(JSON.stringify(p.board)),
     currentPiece: p.currentPiece ? JSON.parse(JSON.stringify(p.currentPiece)) : null,
+    ghostY: p.currentPiece ? getGhostY(p.currentPiece, p.board) : null,
     nextPiece: p.nextPiece ? JSON.parse(JSON.stringify(p.nextPiece)) : null,
     holdPiece: p.holdPiece ? JSON.parse(JSON.stringify(p.holdPiece)) : null,
     canHold: p.canHold,