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/hooks/useWidgets.ts

@@ -10,6 +10,7 @@ import {
 	fetchWidgetInstances,
 	fetchWidgetReferences,
 	updateWidgetInstance,
+	updateWidgetReference,
 } from "../api/widgets";
 import type { WidgetInstanceInput } from "../types";
 
@@ -101,3 +102,19 @@ export function useDetachWidgetReference() {
 		},
 	});
 }
+
+export function useUpdateWidgetReference() {
+	const queryClient = useQueryClient();
+	return useMutation({
+		mutationFn: ({
+			referenceId,
+			sortOrder,
+		}: {
+			referenceId: string;
+			sortOrder: number;
+		}) => updateWidgetReference(referenceId, sortOrder),
+		onSuccess: () => {
+			queryClient.invalidateQueries({ queryKey: ["widgets", "references"] });
+		},
+	});
+}