feat(ui): contextual skeleton loading + list stagger animations
Skeleton loading (5 pages migrated from spinner):
- SettingsPage: tabs + profile + settings cards skeleton
- RolesPage: table header + 6 data rows + assign role skeleton
- MarketplaceHome: filter bar + category pills + 8 product cards
- TrackSearchResults: results count + 8 track card grid
- PlaybackSummary: 3-column stats skeleton
List stagger animations (5 lists):
- New stagger-fade-in CSS keyframe (translateY 8px, 250ms, ease-out)
- 50ms per item delay, capped at 500ms (10+ items render together)
- Applied to: NotificationsPage, PlaylistList, PlaylistTrackList (static),
dashboard TrackList, NotificationMenuList
- Respects prefers-reduced-motion
Co-authored-by: Cursor <cursoragent@cursor.com>
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