feat: per-account cancel + folder message-count progress in log

- orchestrator: per-account cancellable context registry + CancelAccount;
  on cancel, close IMAP connections to unblock in-flight FETCH; account ends
  in 'cancelled' status with a cancelled event
- imapx: CopyDeps.OnFolder callback fires after EXAMINE with the folder's
  message count (before the long fetch) for visibility
- httpapi: POST /tasks/{id}/accounts/{accountId}/cancel
- web: per-row cancel button while running, folder event shows N messages,
  cancelled/done_with_errors status badges

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMHQTtnQtQqL8muAXHr9kd

web/src/components/StatusBadge.tsx

@@ -2,7 +2,7 @@ export function StatusBadge({ status }: { status: string }) {
   const s = (status || 'pending').toLowerCase()
   let cls = 'badge-pending'
   if (s === 'ok' || s === 'done' || s === 'success') cls = 'badge-ok'
-  else if (s === 'fail' || s === 'failed' || s === 'error') cls = 'badge-fail'
+  else if (s === 'fail' || s === 'failed' || s.includes('error')) cls = 'badge-fail'
   else if (s === 'running' || s === 'testing' || s === 'in_progress') cls = 'badge-info'
   return (
     <span className={`badge ${cls}`}>