Add unready toggle in lobby

- Server: Handle unready event to set player ready state to false
- Client: Ready button toggles between READY and UNREADY
- Client: Unready sends unready socket event to server

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

public/js/ui.js

@@ -62,9 +62,15 @@ class UIManager {
   }
 
   handleReady() {
-    network.ready();
-    this.buttons.ready.textContent = 'READY!';
-    this.buttons.ready.disabled = true;
+    if (this.buttons.ready.textContent === 'READY') {
+      network.ready();
+      this.buttons.ready.textContent = 'UNREADY';
+      this.buttons.ready.disabled = false;
+    } else {
+      network.unready();
+      this.buttons.ready.textContent = 'READY';
+      this.buttons.ready.disabled = false;
+    }
   }
 
   handleLeave() {