feat(widgets): add frontend widget runtime (types, API, hooks, registry, components)

PR 3 of 4 for configurable dashboard widgets.

- Add TypeScript widget interfaces (WidgetInstance, WidgetInstanceInput,
  WidgetTypeInfo, WidgetDataResponse).
- Create widget API client for CRUD, registry metadata, and per-widget data.
- Create TanStack Query hooks for instances, data, sources, types, and mutations.
- Create closed frontend widget registry with metadata, source type, refresh
  intervals, and config fields.
- Add six shadcn/ui-based widget components: Jellyfin, Backups, Grafana link,
  Prometheus metric, SSH task output, and static text.
- Add Vitest unit tests for registry metadata.

Verification:
- backend ruff clean; pytest 200 passed
- frontend npm run lint: 0 errors
- frontend npm run build: success
- frontend npm run test -- src/widgets/registry.test.ts: 3 passed

frontend/src/types/index.ts

@@ -443,3 +443,42 @@ export interface PrometheusTarget {
 	labels: Record<string, string>;
 	targets: string[];
 }
+
+export interface WidgetInstance {
+	id: string;
+	addon_id: string;
+	widget_type: string;
+	title: string;
+	config: Record<string, unknown>;
+	enabled: boolean;
+	sort_order: number;
+	created_at: number;
+	updated_at: number;
+}
+
+export interface WidgetInstanceInput {
+	id?: string | null;
+	addon_id: string;
+	widget_type: string;
+	title: string;
+	config: Record<string, unknown>;
+	enabled: boolean;
+	sort_order: number;
+}
+
+export interface WidgetTypeInfo {
+	addon_id: string;
+	widget_type: string;
+	name: string;
+	description: string;
+	source_type: string;
+	config_schema: Record<string, unknown>;
+}
+
+export interface WidgetDataResponse {
+	widget_id: string;
+	widget_type: string;
+	data: Record<string, unknown> | null;
+	error: string | null;
+	fetched_at: number;
+}