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

manifest.json

@@ -1,10 +1,9 @@
 {
   "manifest_version": 3,
   "name": "Tab Shooter",
-  "version": "1.0.0",
+  "version": "1.1.0",
   "description": "Schließe deine offenen Tabs, indem du sie wie ein Raumschiff-Pilot abschießt. Ein kleines Minigame für Tab-Hoarder.",
   "permissions": ["tabs", "storage", "favicon"],
-  "optional_permissions": ["windows"],
   "action": {
     "default_popup": "popup.html",
     "default_title": "Tab Shooter – Tabs abschießen",
@@ -19,6 +18,12 @@
     "48": "icons/icon48.png",
     "128": "icons/icon128.png"
   },
+  "web_accessible_resources": [
+    {
+      "resources": ["game.html", "game.css", "game.js", "engine.js"],
+      "matches": ["<all_urls>"]
+    }
+  ],
   "background": {
     "service_worker": "background.js"
   }