feat(web): top-bar centered nav + Library tab page (replace sidebar)

Operator 2026-06-01: "navigation layout and library sections are
what I'd like to have implemented as it seems better than our
current navbar solution. I think I'd like to have these nav options
moved into the top bar centered."

Top-bar restructure:
- Centered nav (replaces the 192dp left sidebar): Home / Library /
  Discover, with icons + labels. Labels collapse below sm breakpoint
  so the bar stays icon-only on small viewports.
- Right side (search input + user dropdown) unchanged.
- Hamburger button + MobileNavDrawer + the mobileNav store all
  removed - the centered nav lives at all viewport sizes.

Library page restructure (mirrors Android LibraryScreen):
- New routes/library/+layout.svelte renders a tab bar across the
  five Library sub-pages: Artists / Albums / Liked / History /
  Playlists. Active tab gets an accent underline + onSurface text.
- routes/library/+page.server.ts redirects bare /library to
  /library/artists (Android default tab).
- /playlists (list) moved to /library/playlists; old URL gets a 308
  redirect (routes/playlists/+page.server.ts) so existing bookmarks
  land on the new location. /playlists/[id] (detail) is unchanged -
  matches the server API URL shape.

Deleted: Shell's sidebar markup, MobileNavDrawer.{svelte,test.ts},
the mobileNav store, the old routes/playlists/+page.svelte. Shell
test rewritten to assert the new 3-item centered nav; playlists
test moved next to its new +page.svelte and its test-utils import
path updated.

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

@@ -39,30 +39,35 @@ describe('Shell', () => {
     expect(screen.getByText('alice')).toBeInTheDocument();
   });
 
-  test('main nav renders the six primary surfaces in order', () => {
+  test('main nav renders the three primary surfaces in order', () => {
     render(Shell);
-    const labels = ['Home', 'Artists', 'Albums', 'Liked', 'Discover', 'Playlists'];
+    // Library is a parent for Artists / Albums / Liked / History /
+    // Playlists (tabs under /library); only the parent appears here.
+    const labels = ['Home', 'Library', 'Discover'];
     for (const label of labels) {
       expect(screen.getByRole('link', { name: label })).toBeInTheDocument();
     }
     expect(screen.getByRole('link', { name: 'Home' })).toHaveAttribute('href', '/');
-    expect(screen.getByRole('link', { name: 'Artists' })).toHaveAttribute('href', '/library/artists');
-    expect(screen.getByRole('link', { name: 'Albums' })).toHaveAttribute('href', '/library/albums');
+    expect(screen.getByRole('link', { name: 'Library' })).toHaveAttribute('href', '/library');
     expect(screen.getByRole('link', { name: 'Discover' })).toHaveAttribute('href', '/discover');
-    expect(screen.getByRole('link', { name: 'Playlists' })).toHaveAttribute('href', '/playlists');
   });
 
-  test('main nav omits Search, Requests, Hidden, Settings, and Admin', () => {
+  test('main nav omits Artists/Albums/Liked/History/Playlists/Search/Settings/Admin', () => {
     userState.current = { id: '1', username: 'alice', is_admin: true };
     render(Shell);
-    // Search reaches /search via the global SearchInput, Requests is a
-    // tab inside Discover, Hidden lives under Settings, Settings + Admin
-    // live in the username dropdown — none belong on the main nav.
+    // Library sub-pages reach their content via the /library tab bar
+    // (in routes/library/+layout.svelte), not via the global Shell nav.
+    // Search reaches /search via the global SearchInput on the right.
+    // Settings + Admin live in the username dropdown — they're per-
+    // operator chrome, not primary surfaces.
     const navLinks = screen
       .getAllByRole('link')
-      .filter((el) => el.closest('nav') !== null)
+      .filter((el) => el.closest('nav[aria-label="Primary"]') !== null)
       .map((el) => el.textContent?.trim());
-    for (const label of ['Search', 'Requests', 'Hidden', 'Settings', 'Admin']) {
+    for (const label of [
+      'Artists', 'Albums', 'Liked', 'History', 'Playlists',
+      'Search', 'Requests', 'Hidden', 'Settings', 'Admin'
+    ]) {
       expect(navLinks).not.toContain(label);
     }
   });