feat(ui): Sidebar refactor, premium skeletons, ContentFadeIn transitions
- Sidebar: useSidebarNavigation hook, ARIA, token-based layout
- Layout: lg:ml-main-expanded/collapsed (replace arbitrary ml-64)
- TrackCardSkeleton + PlaylistCardSkeleton: KŌDŌ tokens, min-heights for CLS
- ContentFadeIn: 200ms fade-in with --ease-out
- TrackGrid, PlaylistList, LibraryPage: integrate skeletons + fade-in
- Player: player-bar subcomponents, useAudioAnalyser
- Tests: TrackGrid wrapper (QueryClient, ToastProvider)
Co-authored-by: Cursor <cursoragent@cursor.com>
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