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/api/widgets.ts

@@ -0,0 +1,69 @@
+import type {
+	WidgetDataResponse,
+	WidgetInstance,
+	WidgetInstanceInput,
+	WidgetTypeInfo,
+} from "../types";
+
+const API_BASE = "/api";
+
+export async function fetchWidgetSources(): Promise<string[]> {
+	const res = await fetch(`${API_BASE}/widgets/sources`);
+	if (!res.ok) throw new Error("Failed to fetch widget sources");
+	return res.json();
+}
+
+export async function fetchWidgetTypes(): Promise<WidgetTypeInfo[]> {
+	const res = await fetch(`${API_BASE}/widgets/types`);
+	if (!res.ok) throw new Error("Failed to fetch widget types");
+	return res.json();
+}
+
+export async function fetchWidgetInstances(): Promise<WidgetInstance[]> {
+	const res = await fetch(`${API_BASE}/widgets/instances`);
+	if (!res.ok) throw new Error("Failed to fetch widget instances");
+	return res.json();
+}
+
+export async function createWidgetInstance(
+	input: WidgetInstanceInput,
+): Promise<WidgetInstance> {
+	const res = await fetch(`${API_BASE}/widgets/instances`, {
+		method: "POST",
+		headers: { "Content-Type": "application/json" },
+		body: JSON.stringify(input),
+	});
+	if (!res.ok) throw new Error("Failed to create widget instance");
+	return res.json();
+}
+
+export async function updateWidgetInstance(
+	input: WidgetInstanceInput,
+): Promise<WidgetInstance> {
+	if (!input.id) throw new Error("Widget ID is required for update");
+	const res = await fetch(`${API_BASE}/widgets/instances/${input.id}`, {
+		method: "PUT",
+		headers: { "Content-Type": "application/json" },
+		body: JSON.stringify(input),
+	});
+	if (!res.ok) throw new Error("Failed to update widget instance");
+	return res.json();
+}
+
+export async function deleteWidgetInstance(
+	widgetId: string,
+): Promise<{ status: string }> {
+	const res = await fetch(`${API_BASE}/widgets/instances/${widgetId}`, {
+		method: "DELETE",
+	});
+	if (!res.ok) throw new Error("Failed to delete widget instance");
+	return res.json();
+}
+
+export async function fetchWidgetData(
+	widgetId: string,
+): Promise<WidgetDataResponse> {
+	const res = await fetch(`${API_BASE}/widgets/instances/${widgetId}/data`);
+	if (!res.ok) throw new Error("Failed to fetch widget data");
+	return res.json();
+}