[Mobile] Use skeleton placeholders on notifications screen

[dylanjeffers]

Summary

• Replaces the footer spinner on the mobile notifications screen with skeleton notification rows that mirror the shape of real notifications.
• Matches the loading pattern recently introduced for lineups (LineupTileSkeleton in TrackLineup): skeletons render during initial fetch and during pagination, mixed into the same FlashList data array.
• Empty (EmptyNotifications) and error states are preserved — skeletons are suppressed when isError is true, and the empty-state branch is unchanged.

Files

• packages/mobile/src/screens/notifications-screen/NotificationListItemSkeleton.tsx — new skeleton row mirroring the NotificationTile layout (icon + profile picture row + 2 text lines + timestamp placeholder).
• packages/mobile/src/screens/notifications-screen/NotificationList.tsx — interleaves LoadingItem placeholders into the FlashList data; drops the footer spinner.

Test plan

• Open the notifications screen with a slow network — skeleton rows render on initial load instead of a spinner.
• Scroll to fetch the next page — pagination skeletons appear at the bottom while loading.
• Empty state still renders EmptyNotifications when there are no notifications.
• Pull-to-refresh still works without showing initial-load skeletons.
• Error state does not get stuck showing skeletons forever.

🤖 Generated with Claude Code

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