feat: automatic backup on launch, foreground, and photo library change

AutoBackupCoordinator (new):
- @MainActor singleton that owns all auto-backup decisions
- Phase model: idle / checking / autoBackingUp / disconnected
  - idle:          nothing pending or destination not set
  - checking:      reconciliation in progress before decision
  - autoBackingUp: coordinator-triggered BackupEngine run active
  - disconnected:  NAS offline, pending count queued for retry
- Combine subscription on BackupStatusService.$isRefreshing fires
  handleReconciliationComplete() on every reconcile completion — the
  single entry point for all auto-backup decisions
- Combine subscription on LANMonitor.$nasReachable fires
  handleNASReachable() which retries only when phase == .disconnected
- Guards: autoBackupEnabled, quiet hours, chargingOnlyMode + battery state
- canAutoBackup also blocks when engine.job.status.isActive to prevent
  double-start from concurrent photo library change + foreground events

BackupView:
- .task + .onChange(of: scenePhase) now call coordinator.onActive() instead
  of statusService.refresh() directly — coordinator decides whether to also
  start a backup after reconciliation completes
- ringContentID includes coordinator phase suffix → ring animates on
  checking/disconnected transitions
- ringCenterContent case 0: when engine is idle, shows:
  - disconnectedRingView if phase == .disconnected and needBackup > 0
    (wifi.slash icon, "Waiting for NAS", queued count)
  - checkingRingView if phase == .checking (spinner, "Checking…")
  - existing ringMainView otherwise
- ringColor: dim orange (0.35 opacity) when disconnected with pending items
- Pull-to-refresh reconciliation triggers auto-backup via Combine chain

ConnectionStore:
- autoBackupEnabled: Bool, default true, UserDefaults-persisted

SettingsView:
- AUTO BACKUP section at top with autoBackupEnabled toggle

BackgroundTaskManager:
- BGProcessingTask: quiet-hours guard, requiresExternalPower mirrors
  chargingOnlyMode, expiration handler calls engine.cancel() for clean exit
- BGAppRefreshTask: lightweight refresh only (no upload in background refresh)
- Both tasks reschedule themselves before doing any work

AppDelegate:
- UIDevice.current.isBatteryMonitoringEnabled = true for battery checks

NASBackupApp:
- AutoBackupCoordinator.shared injected as @StateObject + environmentObject

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

Shared/Persistence/ConnectionStore.swift

@@ -63,6 +63,11 @@ final class ConnectionStore: ObservableObject {
         didSet { UserDefaults.standard.set(localRetentionDays, forKey: "localRetentionDays") }
     }
 
+    // Auto-backup: default on after first destination is configured
+    @Published var autoBackupEnabled: Bool {
+        didSet { UserDefaults.standard.set(autoBackupEnabled, forKey: "autoBackupEnabled") }
+    }
+
     // Cellular & remote access
     @Published var allowCellularBackup: Bool {
         didSet { UserDefaults.standard.set(allowCellularBackup, forKey: "allowCellularBackup") }
@@ -89,6 +94,7 @@ final class ConnectionStore: ObservableObject {
         onboardingPhotoShown   = UserDefaults.standard.bool(forKey: "onboardingPhotoShown")
         appearanceMode         = AppearanceMode(rawValue: UserDefaults.standard.string(forKey: "appearanceMode") ?? "") ?? .system
         localRetentionDays     = UserDefaults.standard.object(forKey: "localRetentionDays") as? Int ?? 0
+        autoBackupEnabled      = UserDefaults.standard.object(forKey: "autoBackupEnabled") as? Bool ?? true
         allowCellularBackup    = UserDefaults.standard.object(forKey: "allowCellularBackup") as? Bool ?? false
         useTailscaleWhenRemote = UserDefaults.standard.object(forKey: "useTailscaleWhenRemote") as? Bool ?? false
         tailscaleHost          = UserDefaults.standard.string(forKey: "tailscaleHost") ?? ""