Add level selection menu and persistence

Introduce a level selection UI and persistence for unlocked levels. Adds CSS styles and HTML markup for a floating level menu, and JS to load/save highest unlocked level to localStorage (key: mirror-game-highest-unlocked-level). Implements functions to render/toggle the menu, unlock levels on finish, and navigate to arbitrary levels via goToLevel. nextLevel now delegates to goToLevel, and setup calls loadUnlockedLevels() and setupLevelMenu() so the menu reflects progress immediately.

web/templates/view/game.html

@@ -15,6 +15,16 @@
       <button class="win-button" onclick="nextLevel()">Next Level</button>
     </div>
 
+    <aside class="level-menu-shell">
+        <button id="level-menu-toggle" class="level-menu-toggle" type="button" aria-expanded="false" aria-controls="level-menu-panel">
+            Levels
+        </button>
+        <div id="level-menu-panel" class="level-menu-panel" hidden>
+            <h2>Choose Level</h2>
+            <div id="level-menu-list" class="level-menu-list"></div>
+        </div>
+    </aside>
+
     <script>
         atOptions = {
           'key' : '72b6ba1a1c26b9671167b66063c7e699',