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/api.ts

@@ -81,6 +81,9 @@ export const deleteTask = (id: number) => api(`/api/tasks/${id}`, { method: 'DEL
 export const deleteAccount = (taskId: number, accountId: number) =>
   api(`/api/tasks/${taskId}/accounts/${accountId}`, { method: 'DELETE' })
 
+export const cancelAccount = (taskId: number, accountId: number) =>
+  api(`/api/tasks/${taskId}/accounts/${accountId}/cancel`, { method: 'POST' })
+
 export const listTasks = () => api<Task[]>('/api/tasks')
 
 export const getTask = (id: number) => api<TaskDetail>(`/api/tasks/${id}`)