Reusable widgets: reference widgets across dashboards + detach to clone

Widgets configured on one dashboard (e.g., a Grafana service's Overview)
can now be live-referenced on other dashboards. Editing the widget config
updates it everywhere it's referenced. References can be detached into
independent clones.

Backend: new widget_references table (dashboard_scope, widget_id,
sort_order) with ON DELETE CASCADE. CRUD methods + 4 endpoints:
GET/POST /api/widgets/references, DELETE /api/widgets/references/{id},
POST /api/widgets/references/{id}/detach (clones the widget into a
standalone instance, then removes the reference).

Frontend: WidgetConfigDialog gains a dashboardScope prop. When set
(the main Dashboard passes 'main'), the dialog shows:
- Owned + referenced widgets in a combined list, with a link badge on
  references.
- 'Add existing widget' picker: searchable list of ALL widget instances
  not already on this dashboard. Click to create a reference.
- Detach button on references: clones the widget (service_id=NULL) and
  removes the reference.
- Delete on a reference removes the REFERENCE (not the original widget).

Dashboard renders referenced widgets alongside owned widgets.

Detaching a service-bound widget clones it with service_id=NULL — the
clone may need re-binding to a service to render correctly. Named
dashboards don't pass dashboardScope yet (pinned-links-only); when they
gain widget support, the backend already handles any scope string.

282 backend tests pass (+2 reference lifecycle); 127 frontend tests
pass; ruff/eslint/tsc/vite all green.

frontend/src/api/widgets.ts

@@ -6,6 +6,21 @@ import type {
 	WidgetInstanceInput,
 } from "../types";
 
+export interface WidgetReference {
+	id: string;
+	dashboard_scope: string;
+	widget_id: string;
+	sort_order: number;
+	created_at: number;
+	widget: WidgetInstance;
+}
+
+export interface WidgetReferenceInput {
+	dashboard_scope: string;
+	widget_id: string;
+	sort_order?: number;
+}
+
 export async function fetchBuiltinWidgetKinds(): Promise<
 	BuiltinWidgetKindInfo[]
 > {
@@ -46,3 +61,29 @@ export async function fetchWidgetData(
 ): Promise<WidgetDataResponse> {
 	return get<WidgetDataResponse>(`/api/widgets/instances/${widgetId}/data`);
 }
+
+export async function fetchWidgetReferences(
+	dashboardScope: string,
+): Promise<WidgetReference[]> {
+	return get<WidgetReference[]>("/api/widgets/references", {
+		dashboard_scope: dashboardScope,
+	});
+}
+
+export async function createWidgetReference(
+	input: WidgetReferenceInput,
+): Promise<WidgetReference> {
+	return post<WidgetReference>("/api/widgets/references", input);
+}
+
+export async function deleteWidgetReference(
+	referenceId: string,
+): Promise<{ status: string }> {
+	return del<{ status: string }>(`/api/widgets/references/${referenceId}`);
+}
+
+export async function detachWidgetReference(
+	referenceId: string,
+): Promise<WidgetInstance> {
+	return post<WidgetInstance>(`/api/widgets/references/${referenceId}/detach`);
+}