Display a countdown until a retrying task runs again

[marciok]

Summary

For tasks in the retrying state, display a live countdown showing when the next retry attempt will start.

Acceptance criteria

• A retrying task shows the remaining time until its next attempt.
• The countdown updates without a full-page reload.
• The display reaches zero cleanly and disappears or changes when the task leaves retrying.
• Refreshing or reconnecting reconstructs the countdown from authoritative retry timing rather than resetting it.
• The behavior is covered by focused tests.