fix: surface workspace deletion errors

[utpalsinghdev]

Summary

Honcho returns 409 when a workspace still has sessions, with a detail like Cannot delete workspace '<name>': active session(s) remain. Delete all sessions first. The Delete Workspace modal stayed open and showed none of that, so confirm looked like it did nothing.

This PR shows that API detail string in the modal. If the response has no usable detail, it shows a short fallback. Failed deletes keep the modal open so you can cancel or retry. A successful delete still goes to /workspaces. Cancel and reopen clear the last error.

Type

• Bug fix

Test plan

• pnpm lint passes
• pnpm test passes
• @openconcho/web build succeeds
• Tested in browser:
  • Confirming delete on a workspace with sessions showed the 409 detail and left the modal open
  • The confirm button was usable again after the error
  • Cancel then reopen did not keep the old error
  • Deleting a workspace that Honcho accepted closed the modal and returned to the workspace list

Related issues

Closes #73