Fix ready button state after game over

- Enable ready button in resetLobbyState() so players can ready up
- Remove redundant ready button state reset from endGame()
- Ensure ready button is enabled when returning to lobby

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

public/js/ui.js

@@ -81,13 +81,14 @@ class UIManager {
   handleBackToLobby() {
     this.hideGameOver();
     this.resetLobbyState();
+    this.buttons.ready.disabled = false;
     this.showScreen('room');
   }
 
   resetLobbyState() {
     // Reset ready button to default state
     this.buttons.ready.textContent = 'READY';
-    this.buttons.ready.disabled = true;
+    this.buttons.ready.disabled = false;
 
     // Clear player and spectator lists
     this.displays.playerList.innerHTML = '';