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.

backend/src/media_library_viewer_api/widgets/sources.py

@@ -185,21 +185,14 @@ class GrafanaWidgetSource:
                 # Prefer displayName (explicitly set in Grafana), then Prometheus
                 # labels (e.g. {instance: "server:9100", mode: "iowait"}), then
                 # the field name as a last resort.
-                display_name = (
-                    value_field.get("config", {}).get("displayName")
-                    or value_field.get("displayName")
-                )
+                display_name = value_field.get("config", {}).get("displayName") or value_field.get("displayName")
                 frame_labels = value_field.get("labels") or {}
                 if display_name:
                     label = str(display_name)
                 elif frame_labels:
                     # Build a readable label from the Prometheus labels, excluding
                     # redundant ones like __name__.
-                    parts = [
-                        f"{k}={v}"
-                        for k, v in sorted(frame_labels.items())
-                        if not k.startswith("__")
-                    ]
+                    parts = [f"{k}={v}" for k, v in sorted(frame_labels.items()) if not k.startswith("__")]
                     label = " ".join(parts) if parts else "value"
                 else:
                     label = value_field.get("name", "value")