feat: native macOS desktop notifications for task status changes

[cledoux95]

Summary

• Adds native macOS desktop notifications when tasks transition to "ready" or "waiting" states while the app window is unfocused
• Notifications are batched with a 3-second debounce to avoid spamming
• Clicking a notification brings the window to focus and navigates to the relevant task

Changes

• New IPC channels: ShowNotification and NotificationClicked
• New desktopNotifications.ts store module that watches task status transitions
• Electron main process creates native Notification instances and forwards click events back to the renderer

Test plan

• Start tasks, unfocus the app window, and verify notifications appear when tasks complete
• Click a notification and verify it focuses the window and selects the task
• Verify no notifications appear while the window is focused