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.

frontend/src/api/widgets.ts

@@ -87,3 +87,13 @@ export async function detachWidgetReference(
 ): Promise<WidgetInstance> {
 	return post<WidgetInstance>(`/api/widgets/references/${referenceId}/detach`);
 }
+
+export async function updateWidgetReference(
+	referenceId: string,
+	sortOrder: number,
+): Promise<WidgetReference> {
+	return put<WidgetReference>(
+		`/api/widgets/references/${referenceId}?sort_order=${sortOrder}`,
+		{},
+	);
+}