feat: redesign Gallery as photo timeline with sync state, deduplication, and detail view
- GalleryItem: unified model with phone/nas/synced source, sync state enum
- GalleryViewModel: off-actor PHFetchResult + manifest merge, Phone/NAS/All tab filtering, date section grouping, Combine-driven reactivity
- GalleryView: sticky date section headers, 3-col grid, shimmer placeholders, sync dots (green/orange/blue), video duration badge, search bar, context menu (Share, Save to Photos, Delete from Device)
- GalleryDetailView: full-screen swipeable photo viewer (TabView page mode), pinch-to-zoom, double-tap zoom toggle, bottom metadata sheet with file info and actions
- BackupStatusService: expose lastManifest for gallery deduplication without extra NAS connection
- ThumbnailCache: add decodeOffThread static helper for off-actor JPEG decode
- BackupManifest: Sendable conformance for safe Task.detached capture
- ThumbnailCache: synced items load from PHImageManager (instant) instead of NAS
Co-Authored-By: Kutesir <kutesir@provoc.ug>
Co-Authored-By: Sentry <sentry@provoc.ug>
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