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/pages/Dashboard.tsx

@@ -31,7 +31,7 @@ import {
 	useDeleteDashboardShortcut,
 	useSaveDashboardShortcut,
 } from "../hooks/useDashboard";
-import { useWidgetInstances } from "../hooks/useWidgets";
+import { useWidgetInstances, useWidgetReferences } from "../hooks/useWidgets";
 import { useServiceInstances } from "../hooks/useServices";
 import { useIsMobile } from "../hooks/useIsMobile";
 import type {
@@ -452,16 +452,18 @@ export function Dashboard() {
 		undefined,
 		"dashboard",
 	);
+	const { data: widgetReferences = [] } = useWidgetReferences("main");
 	const { data: services = [] } = useServiceInstances();
 	const isMobile = useIsMobile();
 
-	const visibleWidgets = useMemo(
-		() =>
-			widgetInstances
-				.filter((w) => w.enabled)
-				.sort((a, b) => a.sort_order - b.sort_order),
-		[widgetInstances],
-	);
+	const visibleWidgets = useMemo(() => {
+		const refs = widgetReferences
+			.filter((r) => r.widget.enabled)
+			.map((r) => r.widget);
+		return [...widgetInstances, ...refs]
+			.filter((w) => w.enabled)
+			.sort((a, b) => a.sort_order - b.sort_order);
+	}, [widgetInstances, widgetReferences]);
 
 	const mobileSections = useMemo(
 		() => groupWidgetsBySection(visibleWidgets, services),
@@ -592,6 +594,7 @@ export function Dashboard() {
 			<WidgetConfigDialog
 				open={widgetDialogOpen}
 				onClose={() => setWidgetDialogOpen(false)}
+				dashboardScope="main"
 			/>
 		</div>
 	);

frontend/src/pages/__tests__/Dashboard.test.tsx

@@ -23,6 +23,7 @@ vi.mock("../../hooks/useSettings", () => ({
 }));
 vi.mock("../../hooks/useWidgets", () => ({
 	useWidgetInstances: () => ({ data: [] }),
+	useWidgetReferences: () => ({ data: [] }),
 }));
 vi.mock("../../hooks/useServices", () => ({
 	useServiceInstances: () => ({ data: [] }),