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>

Core/Models/GalleryItem.swift

@@ -0,0 +1,47 @@
+import Foundation
+
+struct GalleryItem: Identifiable, Sendable {
+    let id: String
+    let filename: String
+    let creationDate: Date?
+    let duration: TimeInterval?  // non-nil for video
+    let fileSize: Int64?
+    let mediaType: MediaType
+    let source: ItemSource
+
+    enum MediaType: Sendable { case photo, video, unknown }
+
+    enum ItemSource: Sendable {
+        case phone(localIdentifier: String)
+        case nas(path: String)
+        case synced(localIdentifier: String, nasPath: String)
+
+        var localIdentifier: String? {
+            switch self { case .phone(let id), .synced(let id, _): id; case .nas: nil }
+        }
+        var nasPath: String? {
+            switch self { case .nas(let p), .synced(_, let p): p; case .phone: nil }
+        }
+    }
+
+    enum SyncState: Sendable {
+        case phoneOnly   // orange — backed up soon
+        case nasOnly     // blue — on NAS, not this device
+        case synced      // green — confirmed both places
+    }
+
+    var syncState: SyncState {
+        switch source {
+        case .phone:   .phoneOnly
+        case .nas:     .nasOnly
+        case .synced:  .synced
+        }
+    }
+}
+
+struct GallerySection: Identifiable {
+    var id: String        // "2026-05-17" or "unknown"
+    var date: Date
+    var title: String
+    var items: [GalleryItem]
+}