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/Views/Screens/SettingsView.swift

@@ -2596,6 +2596,37 @@ It's better to admit "I need more information" or "I cannot do that" than to fak
                 }
             }
 
+            // Automatic Backup
+            VStack(alignment: .leading, spacing: 6) {
+                sectionHeader("Automatic Backup")
+                formSection {
+                    row("Frequency") {
+                        Picker("", selection: $settingsService.autoBackupFrequency) {
+                            Text("Off").tag("manual")
+                            Text("Daily").tag("daily")
+                            Text("Weekly").tag("weekly")
+                        }
+                        .pickerStyle(.segmented)
+                        .frame(width: 220)
+                    }
+                }
+                Text("When enabled, oAI backs up automatically in the background (checked at launch and hourly while running) — no need to press \"Back Up Now\" yourself.")
+                    .font(.system(size: 13))
+                    .foregroundStyle(.secondary)
+                    .fixedSize(horizontal: false, vertical: true)
+                    .padding(.horizontal, 4)
+            }
+
+            // Favorites Sync
+            VStack(alignment: .leading, spacing: 6) {
+                sectionHeader("Favorite Models")
+                Text("Starred models sync automatically via the same iCloud Drive folder — star a model on one Mac and it appears starred on your others within a launch or two.")
+                    .font(.system(size: 13))
+                    .foregroundStyle(.secondary)
+                    .fixedSize(horizontal: false, vertical: true)
+                    .padding(.horizontal, 4)
+            }
+
             // Actions
             VStack(alignment: .leading, spacing: 6) {
                 sectionHeader("Actions")