feat: add exit game button with confirmation dialog

- Add exit game button in top-left corner (door icon)
- Button only visible during active game phases (not in welcome/setup)
- Confirmation dialog prevents accidental exits
- Clears localStorage state and returns to welcome screen
- Fully responsive with mobile-optimized styling
- Bilingual support with Spanish/English confirmation messages

index.html

@@ -17,6 +17,11 @@
         <span class="language-text">EN</span>
     </button>
 
+    <button id="exit-game" class="exit-game" onclick="confirmExitGame()">
+        <span class="exit-icon">🚪</span>
+        <span class="exit-text" data-i18n="exitGame">Salir de la partida</span>
+    </button>
+
     <div class="container">
         <!-- Welcome screen -->
         <div id="welcome-screen" class="screen active">