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/services/settings_store.py

@@ -165,6 +165,19 @@ class SettingsStore:
                 conn.execute("ALTER TABLE dashboard_widgets ADD COLUMN service_id TEXT")
             if "widget_kind" not in widget_cols:
                 conn.execute("ALTER TABLE dashboard_widgets ADD COLUMN widget_kind TEXT")
+            conn.execute(
+                """
+                CREATE TABLE IF NOT EXISTS widget_references (
+                    id TEXT PRIMARY KEY,
+                    dashboard_scope TEXT NOT NULL,
+                    widget_id TEXT NOT NULL,
+                    sort_order INTEGER DEFAULT 0,
+                    created_at INTEGER NOT NULL,
+                    FOREIGN KEY (widget_id) REFERENCES dashboard_widgets(id) ON DELETE CASCADE
+                )
+                """
+            )
+            conn.execute("CREATE INDEX IF NOT EXISTS idx_widget_references_scope ON widget_references(dashboard_scope)")
             conn.execute("""
                 CREATE TABLE IF NOT EXISTS backup_jobs (
                     id TEXT PRIMARY KEY,
@@ -1491,6 +1504,98 @@ class SettingsStore:
         self.init_schema()
         with self.connect() as conn:
             conn.execute("DELETE FROM dashboard_widgets WHERE id = ?", (widget_id,))
+            conn.execute("DELETE FROM widget_references WHERE widget_id = ?", (widget_id,))
+
+    # ------------------------------------------------------------------
+    # Widget references (live-link widgets across dashboards)
+    # ------------------------------------------------------------------
+
+    def list_widget_references(self, dashboard_scope: str) -> list[dict[str, Any]]:
+        """List widget references for a dashboard scope, joined with widget data."""
+        self.init_schema()
+        with self.connect() as conn:
+            rows = conn.execute(
+                """
+                SELECT wr.id AS ref_id, wr.dashboard_scope, wr.widget_id, wr.sort_order,
+                       wr.created_at AS ref_created_at
+                FROM widget_references wr
+                WHERE wr.dashboard_scope = ?
+                ORDER BY wr.sort_order ASC, wr.created_at ASC
+                """,
+                (dashboard_scope,),
+            ).fetchall()
+        result: list[dict[str, Any]] = []
+        for row in rows:
+            widget = self.get_widget(row["widget_id"])
+            if not widget:
+                continue
+            result.append(
+                {
+                    "id": row["ref_id"],
+                    "dashboard_scope": row["dashboard_scope"],
+                    "widget_id": row["widget_id"],
+                    "sort_order": int(row["sort_order"]),
+                    "created_at": row["ref_created_at"],
+                    "widget": widget,
+                }
+            )
+        return result
+
+    def create_widget_reference(self, dashboard_scope: str, widget_id: str, sort_order: int = 0) -> dict[str, Any]:
+        self.init_schema()
+        widget = self.get_widget(widget_id)
+        if not widget:
+            raise ValueError(f"Widget {widget_id} not found")
+        ref_id = uuid.uuid4().hex[:12]
+        now = int(time.time())
+        with self.connect() as conn:
+            conn.execute(
+                """
+                INSERT INTO widget_references (id, dashboard_scope, widget_id, sort_order, created_at)
+                VALUES (?, ?, ?, ?, ?)
+                """,
+                (ref_id, dashboard_scope, widget_id, sort_order, now),
+            )
+        return {
+            "id": ref_id,
+            "dashboard_scope": dashboard_scope,
+            "widget_id": widget_id,
+            "sort_order": sort_order,
+            "created_at": now,
+            "widget": widget,
+        }
+
+    def delete_widget_reference(self, reference_id: str) -> None:
+        self.init_schema()
+        with self.connect() as conn:
+            conn.execute("DELETE FROM widget_references WHERE id = ?", (reference_id,))
+
+    def detach_widget_reference(self, reference_id: str, dashboard_scope: str) -> dict[str, Any]:
+        """Clone the referenced widget into a new standalone instance owned by the scope."""
+        self.init_schema()
+        with self.connect() as conn:
+            row = conn.execute(
+                "SELECT widget_id FROM widget_references WHERE id = ?",
+                (reference_id,),
+            ).fetchone()
+        if not row:
+            raise ValueError(f"Reference {reference_id} not found")
+        source = self.get_widget(row["widget_id"])
+        if not source:
+            raise ValueError(f"Source widget {row['widget_id']} not found")
+        # Clone: new widget with service_id=NULL (dashboard scope), same config/kind/title.
+        cloned = self.upsert_widget(
+            {
+                "service_id": None,
+                "widget_kind": source["widget_kind"],
+                "title": source["title"],
+                "config": source["config"],
+                "enabled": source["enabled"],
+                "sort_order": source["sort_order"],
+            }
+        )
+        self.delete_widget_reference(reference_id)
+        return cloned
 
     # ------------------------------------------------------------------
     # Service registry