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>

docs/agent-log.md

@@ -4,6 +4,21 @@ What agents have done, newest first. Add an entry for every piece of work: the d
 note or request it was for, what changed, the files touched, the tests run, and anything left
 open or not verified. Keep each entry short; the git history has the details.
 
+## 2026-10-23: Ctrl+W no longer closes the game without asking (Claude Code)
+
+Note: "crash: hoplding crtl and pressing W crashes the whole browser."
+
+- Cause: Ctrl+W is the browser's close-tab shortcut (the whole window if it is the last tab);
+  holding Ctrl for replace while pressing W to fly hits it. Chrome and Brave reserve it, so the
+  page never sees the key and cannot cancel it.
+- `src/ui/input.js`: a `beforeunload` handler makes the browser ask before leaving while
+  `GAME.running`. The `visibilitychange` autosave no longer depends on the audio context
+  existing (it used to return early before any sound had played).
+- `test/play.py`: 2 checks (leaving asks while a world is open, not on the title screen); 120
+  checks pass. All 11 headless suites pass; `t_smooth.py` passed on its rerun (the known flaky
+  half-resolution check failed once at load ~5).
+- Open: the real Ctrl+W prompt can only be seen in a headed browser; the owner checks it.
+
 ## 2026-10-23: build menu hover popup, top-left card hidden while building (Claude Code, with Codex)
 
 Notes: "mouse hover reviels a new info popup…" and "dont need the short description in the build