Clicking a new-mail notification brought spark-rs forward but stayed on
the Calendar screen, so the email opened behind it. Opening from a
notification now leaves the calendar (and closes the sidebar overlay),
as the meeting pop-up's calendar button does in reverse.

Checked by clicking a real toast in a debug build with the calendar
open, before and after. The frontend has no test runner, so there's no
regression test.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>