Ask before leaving the page while a world is open (Ctrl+W closed the game)

Ctrl (replace) + W (fly) hits the browser's close-tab shortcut, which a page
cannot block. A beforeunload prompt now catches it. Autosave on hiding the
tab no longer depends on the audio context.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/ui/input.js

@@ -69,7 +69,11 @@ canvas.addEventListener('wheel', e => {
 }, { passive: false });
 window.addEventListener('resize', () => { if (R.canvas) resizeRender(); });
 document.addEventListener('visibilitychange', () => {
+  if (document.hidden && GAME.running) saveWorld('auto', true);
   if (!AU.ctx) return;
-  if (document.hidden) { if (SET.muteUnfocused) AU.ctx.suspend(); if (GAME.running) saveWorld('auto', true); }
+  if (document.hidden) { if (SET.muteUnfocused) AU.ctx.suspend(); }
   else AU.ctx.resume();
 });
+// Ctrl+W closes the tab and the page cannot stop it, and Ctrl (replace) + W (fly) is an easy slip:
+// while a world is open, ask the browser to confirm leaving
+window.addEventListener('beforeunload', e => { if (GAME.running) { e.preventDefault(); e.returnValue = ''; } });