1.6.0 #9: auto-backup programado a iCloud Drive/Archivos

El BackupService (export JSON v3 lossless a local + iCloud ubiquity, retención,
restore, premium-gated) ya existía de 1.1.0 pero solo era manual. Añadido lo automático:
- BackupService.runScheduledBackupIfNeeded() (config vía UserDefaults, gate por
  backupsEnabled+premium, respeta frecuencia).
- Disparo en foreground (scenePhase .active) en background queue.
- Frecuencia Off/Diario/Semanal/Mensual + fecha del último backup en Settings
  (SettingsViewModel.autoBackupFrequencyDays + updateAutoBackupFrequency).
- Strings en 7 idiomas.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01L2p3gUZRNWW388rWFRjiU7

PortfolioJournal/App/PortfolioJournalApp.swift

@@ -100,6 +100,10 @@ struct PortfolioJournalApp: App {
                 // iCloud changes made on other devices while this device was inactive
                 // are reflected immediately without waiting for a remote-change notification.
                 coreDataStack.refreshFromCloudKit()
+                // Auto-backup: writes a lossless JSON snapshot on the configured cadence.
+                DispatchQueue.global(qos: .utility).async {
+                    BackupService.shared.runScheduledBackupIfNeeded()
+                }
                 NotificationService.shared.scheduleReEngagementNotification()
                 NotificationService.shared.scheduleMonthlyCheckIn()
                 NotificationService.shared.scheduleStreakProtectionReminder()