Extract the beforeunload guard out of useActivityTimer

[gaidheal1]

Summary

useActivityTimer is otherwise a clean, portable timer hook — local tick loop, optimistic state, server sync, auto-stop-at-limit logic. Its one browser dependency is a beforeunload listener that warns the user before closing the tab mid-activity. Extract that into a separate web-only hook so the timer logic itself is platform-neutral.

Part of #569.

Relevant code

frontend/src/hooks/useActivityTimer.ts:336-342:

// Block tab close / refresh / external navigation while timer is active
useEffect(() => {
  if (status !== 'active') return;
  const handler = (e: BeforeUnloadEvent): void => { e.preventDefault(); e.returnValue = ''; };
  window.addEventListener('beforeunload', handler);
  return () => window.removeEventListener('beforeunload', handler);
}, [status]);

Proposed fix

Move this into its own hook (e.g. hooks/useUnloadWarning.ts) taking an active: boolean, and call it from the component that owns the timer UI rather than from inside useActivityTimer. This leaves useActivityTimer with zero window references.

Acceptance criteria

• No window reference remains in useActivityTimer.ts
• The browser still warns on tab close / refresh while an activity timer is active, and does not warn when it isn't
• The extracted hook is a no-op (not a crash) in an environment without window
• useActivityTimer.test.tsx still passes

Notes

There's no native equivalent of beforeunload — protecting an in-progress timer on Android is an AppState / background-task concern and will need designing separately. Extracting the web behaviour now keeps the two concerns from being conflated later.

⚠️ Touches the timer hook, so coordinate with the in-progress timer redesign to avoid conflicting edits. The change is small and additive, but it's in a file that redesign work is likely to be moving around.