Add player names to snake display

Show human-readable player names instead of player IDs in both desktop
and web clients. Player names are now stored in the Snake model and
synchronized across all clients.

Changes:
- Added player_name field to Snake model
- Updated create_snake() to accept player_name parameter
- Desktop client shows "YOU:" or "PlayerName:"
- Web client shows "You (Name)" or "Name"

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

web/game.js

@@ -322,8 +322,8 @@ class GameClient {
             const nameSpan = document.createElement('span');
             nameSpan.className = 'player-name';
             nameSpan.textContent = snake.player_id === this.playerId ?
-                `You (${snake.player_id.substring(0, 8)})` :
-                snake.player_id.substring(0, 8);
+                `You (${snake.player_name})` :
+                snake.player_name;
 
             const scoreSpan = document.createElement('span');
             scoreSpan.className = 'player-score';