Add spectator mode for late-joining players

- Server: Late joiners are added as spectators instead of players
- Server: Send forced-spectator event only to joining spectator (not broadcast)
- Server: Track spectators separately and move them to players after game ends
- Client: Handle forced-spectator event to show all player boards
- Client: Spectators see all boards equally without main/spectator highlighting
- Client: Mobile view shows scrollable vertical list of all boards for spectators
- Fix: All cleared lines are sent as garbage to each opponent (not randomized)

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

public/js/renderer.js

@@ -323,12 +323,17 @@ class TetrisRenderer {
   }
 
   setActivePlayer(playerId) {
-    // Remove main class from all boards
+    // Remove main/spectator classes from all boards
     this.boards.forEach((boardData, id) => {
       boardData.element.classList.remove('active', 'main');
       boardData.element.classList.remove('spectator', 'top-left', 'top-right', 'bottom-left', 'bottom-right');
     });
 
+    // If playerId is null (spectator mode), show all boards equally without highlighting
+    if (playerId === null) {
+      return;
+    }
+
     // Add main class to current player
     const mainBoard = this.boards.get(playerId);
     if (mainBoard) {