fix(tauri): enable and observe tray error notifications

[xuyua9]

Summary

• grant the notification plugin to the main window capability
• preserve delivery failures from tray error notifications in stderr
• keep the existing shared tray/background error path unchanged

Fixes #12.

Validation

• python3 -m json.tool apps/tauri/src-tauri/capabilities/default.json
• git diff --check
• cargo fmt --all -- --check
• cargo check -p sendo

[vercel]

@ming1523 is attempting to deploy a commit to the postigodev's projects Team on Vercel.

A member of the Team first needs to authorize it.