Sync starred models across machines; add automatic backup scheduling

Favorites now push/pull through a small oai_favorites.json file in the
same iCloud Drive folder used by Settings > Backup, reconciled by
last-write-wins timestamp on launch and app-become-active. Also adds
an Off/Daily/Weekly frequency picker so the full settings backup can
run itself (checked at launch and hourly) instead of requiring a
manual "Back Up Now" click every time.

oAI/oAIApp.swift

@@ -43,6 +43,11 @@ struct oAIApp: App {
             await GitSyncService.shared.syncOnStartup()
         }
 
+        // Reconcile starred models with the iCloud copy (cross-machine favorites sync)
+        Task {
+            await BackupService.shared.syncFavoritesOnLaunch()
+        }
+
         // Check for updates in the background
         UpdateCheckService.shared.checkForUpdates()
     }
@@ -56,6 +61,11 @@ struct oAIApp: App {
                     AboutView()
                 }
                 #if os(macOS)
+                .onReceive(NotificationCenter.default.publisher(for: NSApplication.didBecomeActiveNotification)) { _ in
+                    Task {
+                        await BackupService.shared.syncFavoritesOnLaunch()
+                    }
+                }
                 .onReceive(NotificationCenter.default.publisher(for: NSApplication.willTerminateNotification)) { _ in
                     Task { @MainActor in ExternalMCPManager.shared.stopAll() }
                     Task {