feat(web): move Hidden tracks out of main nav, surface from Settings

Hidden tracks (M5b quarantine) is a low-frequency surface — it doesn't
warrant a permanent slot in the main nav. Moves the link under a new
'Library' section card on /settings alongside ListenBrainz config.

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

web/src/lib/components/Shell.test.ts

@@ -41,7 +41,7 @@ describe('Shell', () => {
 
   test('renders nav items in expected order', () => {
     render(Shell);
-    const labels = ['Home', 'Artists', 'Albums', 'Liked', 'Hidden', 'Search',
+    const labels = ['Home', 'Artists', 'Albums', 'Liked', 'Search',
                     'Discover', 'Requests', 'Playlists', 'Settings'];
     for (const label of labels) {
       expect(screen.getByRole('link', { name: label })).toBeInTheDocument();
@@ -55,19 +55,9 @@ describe('Shell', () => {
     expect(screen.getByRole('link', { name: 'Playlists' })).toHaveAttribute('href', '/playlists');
   });
 
-  test('Hidden nav link sits between Liked and Search', () => {
+  test('Hidden link is not in the main nav', () => {
     render(Shell);
-    const hidden = screen.getByRole('link', { name: 'Hidden' });
-    expect(hidden).toHaveAttribute('href', '/library/hidden');
-    const labels = screen
-      .getAllByRole('link')
-      .map((el) => el.textContent?.trim())
-      .filter(Boolean);
-    const idxLiked  = labels.indexOf('Liked');
-    const idxHidden = labels.indexOf('Hidden');
-    const idxSearch = labels.indexOf('Search');
-    expect(idxLiked).toBeLessThan(idxHidden);
-    expect(idxHidden).toBeLessThan(idxSearch);
+    expect(screen.queryByRole('link', { name: 'Hidden' })).not.toBeInTheDocument();
   });
 
   test('non-admin users do not see the Admin nav link', () => {