Add live backup status reconciliation with NAS manifest

BackupStatusService (new singleton, ObservableObject, PHPhotoLibraryChangeObserver):
- Loads cached BackupStatusSnapshot instantly from UserDefaults on init
- Full reconcile: fetch phone assets → connect NAS → load/build manifest → compare
  → enforce invariant (alreadySafe ≤ phoneTotal), persist result
- Debounced (30s) for photo library changes; force=true bypasses debounce
- If NAS unreachable: keeps last cached numbers, marks connectionState = .offline
- PHPhotoLibraryChangeObserver triggers refresh on any library change

BackupManifest (new):
- Stored at {remotePath}/.kisani.json on NAS
- Indexed by localIdentifier (PHAsset stable ID); filename fallback for legacy entries
- Built from directory listing if manifest missing (bootstrap for existing backups)
- merged/updated after each backup run via NASTransferProtocol.writeData

BackupStatusSnapshot (new):
- Single source of truth: phoneTotal, alreadySafe, needBackup (derived), nasArchiveTotal,
  lastCheckedAt, connectionState
- Invariant enforced in service: alreadySafe = min(safe, phoneTotal)

Protocol / services:
- NASTransferProtocol: adds writeData(_ data: Data, to remotePath: String)
- SMBService: implements writeData via temp file + SMBClient.upload
- SFTPService: implements writeData via SFTP ByteBuffer write

BackupEngine:
- Tracks ManifestEntry per successful upload during backup loop
- After backup: calls BackupStatusService.refreshAfterBackup(entries:connection:)
  which applies optimistic UI update then writes manifest + triggers reconcile

BackupView:
- Reads all stats from BackupStatusService.snapshot (not vm/nasFileCount)
- Stats labels: "NAS Archive" / "On iPhone" / "Need Backup" / "Already Safe"
- Live refresh triggers: .task (force), scenePhase.active (force),
  nasReachable change (force), remotePath change (force), backup completed (+2s)
- Subtle status row below stats: "Checking…" spinner or "Updated X ago" with
  wifi-slash icon when NAS offline; tap refresh button for forced reconcile
- AppMenuView sheet now correctly passes engine EnvironmentObject

BackupViewModel: stripped to auth-only (photosAuthStatus + requestPhotosAccess)

Co-Authored-By: Kutesir <kutesir@provoc.ug>
Co-Authored-By: Sentry <sentry@provoc.ug>

Features/Backup/BackupViewModel.swift

@@ -3,42 +3,9 @@ import Photos
 
 @MainActor
 final class BackupViewModel: ObservableObject {
-    @Published var totalPhotoCount: Int = 0
-    @Published var alreadySafeCount: Int = 0
-    @Published var notBackedUpCount: Int = 0
     @Published var photosAuthStatus: PHAuthorizationStatus = PHPhotoLibrary.authorizationStatus(for: .readWrite)
 
     private let photoService = PhotoLibraryService()
-    private var fetchedAssets: [PhotoAsset] = []
-
-    func loadPhotoCount(filter: BackupFilter) {
-        let assets = photoService.fetchAssets(filter: filter)
-        fetchedAssets = assets
-        totalPhotoCount = assets.count
-        // Conservative initial state until NAS scan completes
-        alreadySafeCount = 0
-        notBackedUpCount = assets.count
-    }
-
-    // Called after NAS directory listing loads.
-    // Matches by lowercased filename — same heuristic BackupEngine uses for skip checks.
-    // This gives an accurate pre-scan "Already Safe / Not Backed Up" split before
-    // the user ever taps Start Backup.
-    func compareWithNAS(nasItems: [NASItem]) {
-        let nasFilenames = Set(
-            nasItems
-                .filter { !$0.isDirectory }
-                .map { $0.name.lowercased() }
-        )
-        var safe = 0
-        for asset in fetchedAssets {
-            if nasFilenames.contains(asset.filename.lowercased()) {
-                safe += 1
-            }
-        }
-        alreadySafeCount = safe
-        notBackedUpCount = totalPhotoCount - safe
-    }
 
     func requestPhotosAccess() async {
         photosAuthStatus = await photoService.requestAuthorization()