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/components/__tests__/WidgetConfigDialog.test.tsx

@@ -18,8 +18,12 @@ function setMatchMedia(matches: boolean) {
 
 vi.mock("../../hooks/useWidgets", () => ({
 	useWidgetInstances: () => ({ data: [] }),
+	useWidgetReferences: () => ({ data: [] }),
 	useSaveWidgetInstance: () => ({ mutateAsync: vi.fn(), isPending: false }),
 	useDeleteWidgetInstance: () => ({ mutateAsync: vi.fn(), isPending: false }),
+	useCreateWidgetReference: () => ({ mutateAsync: vi.fn() }),
+	useDeleteWidgetReference: () => ({ mutateAsync: vi.fn() }),
+	useDetachWidgetReference: () => ({ mutateAsync: vi.fn() }),
 }));
 
 vi.mock("../../hooks/useServices", () => ({