fix(HAI-018): complete Step 2 — add client-side retry for fetchTaskDetail

- Added withRetry wrapper in api.ts (1 retry, 200ms delay)
- fetchTaskDetail now retries transient 500s transparently
- TaskCard shows 'Failed to load task details' instead of raw error
- Added api.test.ts with success, retry-success, and exhaustion tests

packages/dashboard/app/components/TaskCard.tsx

@@ -47,7 +47,7 @@ export function TaskCard({ task, queued, onOpenDetail, addToast }: TaskCardProps
       const detail = await fetchTaskDetail(task.id);
       onOpenDetail(detail);
     } catch (err: any) {
-      addToast(err.message, "error");
+      addToast("Failed to load task details", "error");
     }
   }, [task.id, onOpenDetail, addToast]);