macOS notification clicks do not route to the target message

**Describe the bug**
On macOS, clicking an actionable notification (eg. new message in a chat) re-focuses Buzz but does not route the UI properly to the specific message. Buzz remains in the channel that was open before the click.

**Steps to reproduce**
1. Prompt an agent in channel X.
2. Switch to channel Y before generation completes.
3. Focus another macOS application.
4. Wait for the completion notification.
5. Click the notification body.

**Expected behavior**
Buzz should activate, open channel X, and navigate to the generated message.

**Version and platform**
Buzz version: 0.4.26
OS: macOS 27.0.0

**Logs / additional context**
No error is shown. The notification target is sent as Tauri extra metadata, but the macOS notification backend does not return that metadata with the click action.

Start by tracing the Tauri extra metadata from notification creation through the macOS notification click-action handling. Reproduce the channel-switch scenario and verify that clicking the completion notification activates Buzz, opens channel X, and navigates to the generated message.

Written by the indexing model from the issue text.