Note

Add OS notifications for completed and failed background tasks

• Adds a nativeNotifications utility library (nativeNotifications.ts) with functions to check permission, request permission, detect app backgrounding, and dispatch native notifications.
• Notifications fire once per turn when a thread's running task transitions to completed or failed while the app is backgrounded, using the thread title or last error as the message.
• Adds a useNotification hook to track and request Notification API permission state, refreshing on window focus.
• Adds an enableNotifications toggle (default true) to app settings, with a settings UI for enabling/disabling notifications, viewing permission status, requesting browser permission, and sending a test notification.
• Behavioral Change: new installs default to enableNotifications: true; users will be prompted for browser notification permission when they visit settings.

^{Macroscope summarized 0a1e63b.}