feat(FN-3897): add blocker fan-out hook, badge, and detail modal section

Implemented the blocker fan-out feature across the dashboard: added a `useBlockerFanout` hook to track blocking relationships, rendered a fan-out badge on task cards, and added a blocking section to the task detail modal, with tests covering the hook and responsive modal behavior.

Fusion-Task-Id: FN-3897

packages/dashboard/app/components/TaskDetailModal.css

@@ -522,6 +522,11 @@
   position: relative;
 }
 
+.detail-blocking-item--stale {
+  color: var(--color-warning);
+  font-size: 0.75rem;
+}
+
 .modal-actions-spacer {
   flex: 1;
 }