Frontend: admin panel Kick button + confirm dialog consuming disconnect endpoint

[birme]

Parent: #638 (admin control panel, epic #280). Broken out by daily-backlog-pr triage on 2026-08-26.

Add a per-participant Kick button in the admin/control panel (user-list.tsx area, mirroring the existing MuteParticipantButton + confirm pattern) that calls the new backend force-disconnect endpoint via api.ts for the selected sessionId. Include a confirmation dialog and error handling.

Depends on the backend force-disconnect endpoint (Eyevinn/intercom-manager sub-issue).