Follow-ups: reference reorder, detach service_id, named-dashboard widgets

Three reusable-widget follow-up fixes:

1. Reference sort_order independently reorderable. Reordering a
   referenced widget now updates the widget_references.sort_order (per-
   dashboard), not the shared widget instance sort_order. New backend
   update_widget_reference method + PUT /api/widgets/references/{id}
   endpoint. Frontend moveInstance checks _ref_id to choose the right
   mutation (updateRef for references, saveWidget for owned).

2. Detach preserves service_id. detach_widget_reference now copies the
   original widget's service_id into the clone, so service-bound widgets
   (Grafana chart, Jellyfin activity) continue to render after detach.

3. Named dashboards support widget references. NamedDashboardPage
   fetches useWidgetReferences('named:<slug>') and renders them via
   WidgetInstanceCard alongside pinned links. 'Edit widgets' button
   opens WidgetConfigDialog with dashboardScope='named:<slug>'.

Also: removed useMemo on combinedWidgets in WidgetConfigDialog to fix
a react-hooks/preserve-manual-memoization lint error (the React Compiler
ESLint plugin couldn't verify the spread+sort memoization).

283 backend tests pass (+1 update_reference test); 128 frontend tests
pass; ruff clean; 0 lint errors.

backend/src/media_library_viewer_api/routers/widgets.py

@@ -269,6 +269,19 @@ def delete_reference(
     return {"status": "deleted"}
 
 
+@router.put("/references/{reference_id}")
+def update_reference(
+    reference_id: str,
+    sort_order: int,
+    store: SettingsStore = Depends(get_settings_store),
+) -> dict[str, Any]:
+    """Update a widget reference's sort_order (per-dashboard reordering)."""
+    try:
+        return store.update_widget_reference(reference_id, sort_order)
+    except ValueError as exc:
+        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail=str(exc)) from exc
+
+
 @router.post("/references/{reference_id}/detach")
 def detach_reference(
     reference_id: str,