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/Services/BackupService.swift

@@ -34,6 +34,15 @@ struct BackupManifest: Codable {
     let credentials: [String: String]?
 }
 
+// MARK: - FavoritesPayload
+
+/// Small standalone file (separate from the full settings backup) so starring a model
+/// syncs near-instantly across machines instead of waiting for the next full backup.
+struct FavoritesPayload: Codable {
+    let updatedAt: String
+    let ids: [String]
+}
+
 // MARK: - BackupService
 
 @Observable
@@ -51,6 +60,8 @@ final class BackupService {
     /// URL of the last backup file
     var lastBackupURL: URL?
 
+    private var autoBackupTimer: Timer?
+
     // Keys excluded from backup — encrypted_ prefix + internal migration flags
     private static let excludedKeys: Set<String> = [
         "encrypted_openrouterAPIKey",
@@ -71,6 +82,7 @@ final class BackupService {
 
     private init() {
         checkForExistingBackup()
+        startAutoBackupTimer()
     }
 
     // MARK: - iCloud Path Resolution
@@ -176,6 +188,79 @@ final class BackupService {
         log.info("Restored \(manifest.settings.count) settings from backup (v\(manifest.version))")
     }
 
+    // MARK: - Favorite Models Sync
+
+    private func favoritesFileURL() -> URL {
+        resolveBackupDirectory().appendingPathComponent("oai_favorites.json")
+    }
+
+    /// Write the current local favorites to iCloud Drive. Called whenever a favorite is toggled.
+    func pushFavorites() async {
+        let settings = SettingsService.shared
+        let payload = FavoritesPayload(
+            updatedAt: settings.favoriteModelsUpdatedAt,
+            ids: settings.favoriteModelIds.sorted()
+        )
+        guard let data = try? JSONEncoder().encode(payload) else { return }
+        try? data.write(to: favoritesFileURL(), options: .atomic)
+        log.debug("Pushed \(payload.ids.count) favorite model(s) to iCloud")
+    }
+
+    /// Reconcile local favorites with the iCloud copy using last-write-wins (by timestamp).
+    /// Call on launch (and optionally on app-become-active) to pick up changes from other machines.
+    func syncFavoritesOnLaunch() async {
+        let settings = SettingsService.shared
+        let fileURL = favoritesFileURL()
+
+        guard let data = try? Data(contentsOf: fileURL),
+              let remote = try? JSONDecoder().decode(FavoritesPayload.self, from: data) else {
+            // No remote copy yet — push local state (may be empty, that's fine).
+            await pushFavorites()
+            return
+        }
+
+        let localUpdatedAt = settings.favoriteModelsUpdatedAt
+        if remote.updatedAt > localUpdatedAt {
+            settings.favoriteModelIds = Set(remote.ids)
+            settings.favoriteModelsUpdatedAt = remote.updatedAt
+            log.info("Applied \(remote.ids.count) favorite model(s) from iCloud (remote was newer)")
+        } else if localUpdatedAt > remote.updatedAt {
+            await pushFavorites()
+        }
+        // Equal timestamps: already in sync, nothing to do.
+    }
+
+    // MARK: - Automatic Backup
+
+    private static let dailyInterval: TimeInterval = 24 * 3600
+    private static let weeklyInterval: TimeInterval = 7 * 24 * 3600
+
+    /// Checked once at launch and hourly thereafter while the app is running.
+    private func startAutoBackupTimer() {
+        Task { await checkAndPerformAutoBackupIfDue() }
+        autoBackupTimer = Timer.scheduledTimer(withTimeInterval: 3600, repeats: true) { [weak self] _ in
+            Task { await self?.checkAndPerformAutoBackupIfDue() }
+        }
+    }
+
+    func checkAndPerformAutoBackupIfDue() async {
+        let frequency = SettingsService.shared.autoBackupFrequency
+        let interval: TimeInterval
+        switch frequency {
+        case "daily": interval = Self.dailyInterval
+        case "weekly": interval = Self.weeklyInterval
+        default: return // "manual" — user triggers backups by hand
+        }
+
+        checkForExistingBackup()
+        if let last = lastBackupDate, Date().timeIntervalSince(last) < interval {
+            return // not due yet
+        }
+
+        log.info("Automatic backup (\(frequency, privacy: .public)) is due — backing up now")
+        _ = try? await exportSettings()
+    }
+
     // MARK: - Helpers
 
     private func appVersion() -> String {