chore(ui): delete unused usePageVisible hook

Added as a reusable primitive during Plan 03 Task 9, but the intended
consumer (NotificationBell live-region refresh) was removed during
code review, leaving the hook unused. Delete it — YAGNI; reintroduce
when a real consumer shows up.

Verified upstream impact (gitnexus): 0 callers, LOW risk.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ui/src/hooks/usePageVisible.test.ts

@@ -1,30 +0,0 @@
-import { describe, it, expect, beforeEach } from 'vitest';
-import { renderHook, act } from '@testing-library/react';
-import { usePageVisible } from './usePageVisible';
-
-describe('usePageVisible', () => {
-  beforeEach(() => {
-    Object.defineProperty(document, 'visibilityState', {
-      value: 'visible',
-      configurable: true,
-      writable: true,
-    });
-  });
-
-  it('returns true when visible, false when hidden', () => {
-    const { result } = renderHook(() => usePageVisible());
-    expect(result.current).toBe(true);
-
-    act(() => {
-      Object.defineProperty(document, 'visibilityState', { value: 'hidden', configurable: true });
-      document.dispatchEvent(new Event('visibilitychange'));
-    });
-    expect(result.current).toBe(false);
-
-    act(() => {
-      Object.defineProperty(document, 'visibilityState', { value: 'visible', configurable: true });
-      document.dispatchEvent(new Event('visibilitychange'));
-    });
-    expect(result.current).toBe(true);
-  });
-});

ui/src/hooks/usePageVisible.ts

@@ -1,22 +0,0 @@
-import { useEffect, useState } from 'react';
-
-/**
- * Tracks Page Visibility API state for the current document.
- *
- * Returns `true` when the tab is visible, `false` when hidden. Useful for
- * pausing work (polling, animations, expensive DOM effects) while the tab
- * is backgrounded. SSR-safe: defaults to `true` when `document` is undefined.
- */
-export function usePageVisible(): boolean {
-  const [visible, setVisible] = useState(() =>
-    typeof document === 'undefined' ? true : document.visibilityState === 'visible',
-  );
-
-  useEffect(() => {
-    const onChange = () => setVisible(document.visibilityState === 'visible');
-    document.addEventListener('visibilitychange', onChange);
-    return () => document.removeEventListener('visibilitychange', onChange);
-  }, []);
-
-  return visible;
-}