feat: save-on-pause via visibilitychange and beforeunload (#57)

Auto-save game state when the browser tab is hidden (visibilitychange)
or closed/navigated away (beforeunload). Prevents data loss on mobile
where tab switching can kill the page without a save interval firing.

Part of epic #57 Night of Polish — Task 6: Mobile Polish.

js/main.js

@@ -99,3 +99,13 @@ window.addEventListener('keydown', function (e) {
         saveGame();
     }
 });
+
+// Save-on-pause: auto-save when tab is hidden or closed (#57 Mobile Polish)
+document.addEventListener('visibilitychange', function () {
+    if (document.hidden) {
+        saveGame();
+    }
+});
+window.addEventListener('beforeunload', function () {
+    saveGame();
+});