[Feature request] Scheduled tasks: run once at a specific date #403

Scheduled tasks (#300) only repeat: daily, weekdays, weekly.
There is no way to run a task once, on a specific date, for example on 27 October 2026.

Add a once cadence:
a local date (YYYY-MM-DD) plus a time (HH:MM).
It fires a single time and never again.

Scope:

• Schedule dialog: a fourth "Once" option with a date field.
• Scheduled view: "Once on {date} at {time}", and "Done" once it has run.
• CLI and MCP: termic schedule set --cadence once --date 2026-10-27 --time 09:00, and a date parameter on schedule_set.
• A one-off whose time passed while the app was closed is recorded as missed, or run once if catch-up is on.
• Editing the date re-arms a finished one-off.

Approved by the maintainer.