Reset lobby UI state when returning from game over

- Add resetLobbyState() to UIManager to clear player lists and reset ready button
- Call resetLobbyState() when player-joined event has null player (game over reset)
- Reset ready button state in endGame() before showing game over screen

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

public/js/app.js

@@ -27,6 +27,10 @@ function setupNetworkListeners() {
     if (ui.screens.room.classList.contains('active')) {
       ui.showScreen('lobby');
     }
+    // Reset lobby state when returning from game over (player is null indicates game over reset)
+    if (!player) {
+      ui.resetLobbyState();
+    }
   });
 
   // Player left lobby
@@ -225,6 +229,10 @@ function endGame(states) {
   network.isSpectator = false;
   ui.hideSpectatorMode();
 
+  // Reset ready button state
+  ui.buttons.ready.textContent = 'READY';
+  ui.buttons.ready.disabled = true;
+
   ui.showGameOver(winner, scores);
 }