feat: add notifications and unread state for sessions

[dakl]

Summary

• Unread state: Sessions that transition to waiting_for_input while not the active session get marked as unread with a persistent chat bubble in the sidebar. The bubble clears after viewing the session for 1.5s, or when the session resumes/archives.
• Cmd+U shortcut: Manually marks the active session as unread (hold Cmd to see a "U" badge hint on the active session).
• macOS notifications: When the window is not focused and a session starts waiting for input, a native macOS notification fires. Clicking it focuses the window and navigates to that session.
• Settings toggle: A new "Notifications" section in Settings lets you enable/disable macOS notifications.

Test plan

• 10 new tests for unread state in sessionStore.test.ts
• 4 new tests for isMarkUnreadShortcut in useGlobalShortcuts.test.ts
• All 56 renderer/shared tests pass
• Biome lint clean
• Manual: Start two sessions, switch to one, let the other finish → bubble appears on background session
• Manual: Click the unread session, wait 1.5s → bubble clears
• Manual: Click away before 1.5s → bubble persists
• Manual: Cmd+U on active session → marks it unread
• Manual: Hold Cmd → "U" badge shows on active session
• Manual: Minimize window, let session finish → macOS notification appears
• Manual: Click notification → window focuses, switches to that session
• Manual: Toggle notifications off in settings → no more notifications

🤖 Generated with Claude Code

[greptile-apps]

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