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>

Features/Settings/SettingsView.swift

@@ -12,6 +12,7 @@ struct SettingsView: View {
             ScrollView {
                 VStack(spacing: AppTheme.sectionGap) {
                     appearanceSection
+                    autoBackupSection
                     LANTriggerSection()
                     networkStatusSection
                     mediaSection
@@ -41,6 +42,26 @@ struct SettingsView: View {
         }
     }
 
+    // MARK: — Auto Backup
+
+    private var autoBackupSection: some View {
+        VStack(alignment: .leading, spacing: 8) {
+            sectionLabel("AUTO BACKUP")
+
+            VStack(spacing: 0) {
+                ToggleRow(
+                    icon: "arrow.clockwise.icloud",
+                    title: "Auto backup",
+                    subtitle: "Back up new photos automatically when the app opens",
+                    isOn: $store.autoBackupEnabled
+                )
+            }
+            .background(AppTheme.surfaceRaised)
+            .clipShape(RoundedRectangle(cornerRadius: AppTheme.radius, style: .continuous))
+            .shadow(color: AppTheme.cardShadow, radius: 12, x: 0, y: 2)
+        }
+    }
+
     // MARK: — Appearance
 
     private var appearanceSection: some View {