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

internal/httpapi/run.go

@@ -85,6 +85,19 @@ func (s *Server) handleRun(w http.ResponseWriter, r *http.Request) {
 	writeJSON(w, http.StatusAccepted, map[string]int64{"run_id": runID})
 }
 
+func (s *Server) handleCancelAccount(w http.ResponseWriter, r *http.Request) {
+	accID, err := pathID(r, "accountId")
+	if err != nil {
+		http.Error(w, "bad account id", http.StatusBadRequest)
+		return
+	}
+	if !s.orch.CancelAccount(accID) {
+		http.Error(w, "account is not running", http.StatusConflict)
+		return
+	}
+	w.WriteHeader(http.StatusAccepted)
+}
+
 func (s *Server) handleDeleteTask(w http.ResponseWriter, r *http.Request) {
 	id, err := pathID(r, "id")
 	if err != nil {