feat(web): drop "Remove from library" from the track kebab

No single-click destructive action belongs in the kebab. Removing the item
orphaned its whole path (RemoveTrackPopover was its only caller, and the
admin/tracks API client was the popover's only caller), so per the repo's
no-dead-code convention the chain is fully removed: the menu item + its
admin/isAdmin plumbing in TrackMenu, RemoveTrackPopover(.svelte/.test),
src/lib/api/admin/tracks(.ts/.test), and the now-needless transitive mocks
in the CompactTrackCard / PlaylistTrackRow / playlist specs.

The kebab is now an 8-item, admin-agnostic menu. The DELETE /api/admin/tracks
server endpoint is untouched — a future safer admin surface can rebind it.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/src/routes/playlists/[id]/playlist.test.ts

@@ -41,7 +41,6 @@ vi.mock('$lib/api/likes', () => ({
   unlikeEntity: vi.fn()
 }));
 vi.mock('$lib/api/quarantine', () => emptyQuarantineMock());
-vi.mock('$lib/api/admin/tracks', () => ({ removeTrack: vi.fn() }));
 
 import PlaylistDetailPage from './+page.svelte';
 import { createPlaylistQuery, deletePlaylist, refreshSystem } from '$lib/api/playlists';