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/stores/mobileNav.svelte.ts

@@ -1,22 +0,0 @@
-// Open/close state for the off-canvas mobile navigation drawer.
-// Mounted once from Shell.svelte; toggled by the hamburger button.
-
-let _open = $state(false);
-
-export const mobileNav = {
-  get value(): boolean {
-    return _open;
-  }
-};
-
-export function openMobileNav(): void {
-  _open = true;
-}
-
-export function closeMobileNav(): void {
-  _open = false;
-}
-
-export function toggleMobileNav(): void {
-  _open = !_open;
-}