feat: rerun a selected step and all downstream steps

[yohamta0]

Split from #876.

Dagu supports retrying a single selected step through the UI, CLI, and API. The remaining request is an explicit mode that reruns the selected step together with every downstream dependent step while preserving unrelated branches.

Expected behavior

• The UI offers a choice between rerunning only the selected step and rerunning it with downstream steps.
• The API and CLI expose the same explicit choice.
• Only the selected step and its reachable descendants are reset.
• Unrelated branches retain their existing status.
• The behavior works regardless of whether the selected step previously succeeded, failed, or was skipped.