feat: add fullscreen mode and extract shared game engine

- Add new fullscreen game mode playable in a dedicated browser tab
- Extract shared game logic into `engine.js` to be used by both popup and fullscreen modes
- Implement self-protection to prevent closing the game tab or popup tab
- Update documentation and bump version to 1.1.0

popup.html

@@ -27,6 +27,7 @@
       </div>
 
       <button id="start-btn" class="primary-btn">SPIEL STARTEN</button>
+      <button id="fullscreen-btn" class="secondary-btn">🖥️ Im Vollbild spielen</button>
 
       <div class="highscore-display">
         <span class="hs-label">🏆 Highscore</span>
@@ -91,6 +92,6 @@
     </div>
   </div>
 
-  <script src="popup.js"></script>
+  <script type="module" src="popup.js"></script>
 </body>
 </html>