Migrate agent node start/stop/restart actions from Control Panel to WebUI

[ironAiken2]

Summary

Migrate the agent node lifecycle controls (start / stop / restart) that currently live in the Control Panel into the Backend.AI WebUI, so administrators can manage agent nodes directly from the WebUI without leaving the application.

Background

Today, starting, stopping, and restarting an agent node must be done through the Control Panel. These operations should be available in the WebUI agent management view to provide a unified administration experience.

Scope

• Add Start, Stop, and Restart actions for agent nodes in the WebUI.
• Surface the actions in the appropriate agent node management view (e.g. the agent list / detail).
• Apply destructive-action confirmation UX where appropriate (stop/restart of a running node).
• Reflect the resulting agent node status after each action.
• Handle loading / error states for each async action.

Out of Scope

• Backend/Manager API changes (assume the required API exists or is tracked separately).

Acceptance Criteria

• Admin can start an agent node from the WebUI.
• Admin can stop an agent node from the WebUI.
• Admin can restart an agent node from the WebUI.
• Each action shows proper loading/feedback and reflects the updated node status.
• Confirmation UX follows the project convention for impactful actions.

JIRA Issue: FR-3230