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/hooks/useWidgets.ts

@@ -0,0 +1,64 @@
+import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
+import {
+	createWidgetInstance,
+	deleteWidgetInstance,
+	fetchWidgetData,
+	fetchWidgetInstances,
+	fetchWidgetSources,
+	fetchWidgetTypes,
+	updateWidgetInstance,
+} from "../api/widgets";
+import type { WidgetInstanceInput } from "../types";
+
+export function useWidgetInstances() {
+	return useQuery({
+		queryKey: ["widgets", "instances"],
+		queryFn: fetchWidgetInstances,
+		refetchInterval: 60_000,
+	});
+}
+
+export function useWidgetData(widgetId: string, refreshInterval: number) {
+	return useQuery({
+		queryKey: ["widgets", "data", widgetId],
+		queryFn: () => fetchWidgetData(widgetId),
+		refetchInterval: refreshInterval || false,
+		enabled: !!widgetId,
+		retry: 1,
+	});
+}
+
+export function useSaveWidgetInstance() {
+	const queryClient = useQueryClient();
+	return useMutation({
+		mutationFn: (input: WidgetInstanceInput) =>
+			input.id ? updateWidgetInstance(input) : createWidgetInstance(input),
+		onSuccess: () => {
+			queryClient.invalidateQueries({ queryKey: ["widgets", "instances"] });
+		},
+	});
+}
+
+export function useDeleteWidgetInstance() {
+	const queryClient = useQueryClient();
+	return useMutation({
+		mutationFn: (widgetId: string) => deleteWidgetInstance(widgetId),
+		onSuccess: () => {
+			queryClient.invalidateQueries({ queryKey: ["widgets", "instances"] });
+		},
+	});
+}
+
+export function useWidgetSources() {
+	return useQuery({
+		queryKey: ["widgets", "sources"],
+		queryFn: fetchWidgetSources,
+	});
+}
+
+export function useWidgetTypes() {
+	return useQuery({
+		queryKey: ["widgets", "types"],
+		queryFn: fetchWidgetTypes,
+	});
+}