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>

Services/ThumbnailCache.swift

@@ -39,6 +39,14 @@ final class ThumbnailCache {
     }
 }
 
+extension ThumbnailCache {
+    static func decodeOffThread(data: Data, size: CGFloat) async -> UIImage? {
+        await Task.detached(priority: .userInitiated) {
+            UIImage(data: data)?.thumbnailScaled(to: size)
+        }.value
+    }
+}
+
 extension UIImage {
     func thumbnailScaled(to size: CGFloat) -> UIImage {
         let scale = min(size / self.size.width, size / self.size.height)