feat(services): frontend services runtime and widget rebind

PR 3 of 4 for the runtime service registry change.

- Add service + new-shape widget TypeScript types; widgets carry service_id
  + widget_kind (service-bound) or null (built-in).
- Add services API client + TanStack Query hooks; reconcile the widget API
  client/hooks to the new endpoints (remove sources/types; add builtin kinds).
- Add closed frontend service registry (integrations/registry.ts) mirroring the
  backend, with resolveWidget(widget, services) mapping a widget to its
  component + refresh interval.
- Add ServicePage at /services/:serviceType/:serviceId with config view,
  empty-on-edit secret inputs + 'set' badges, enable toggle, delete, and the
  service's widget-kind list.
- Register /services/:serviceType/:serviceId in App.tsx.
- Reconcile the six widget components to refreshIntervalMs + description props;
  rewrite WidgetConfigDialog around a service -> widget-kind picker.
- Update Dashboard test; add integrations/registry.test.ts.

Verification: frontend lint 0 errors, build success, 70 tests passed; backend
ruff clean, 222 tests passed.

frontend/src/api/widgets.ts

@@ -1,21 +1,15 @@
 import type {
+	BuiltinWidgetKindInfo,
 	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");
+export async function fetchBuiltinWidgetKinds(): Promise<BuiltinWidgetKindInfo[]> {
+	const res = await fetch(`${API_BASE}/widgets/builtin`);
+	if (!res.ok) throw new Error("Failed to fetch built-in widget kinds");
 	return res.json();
 }