feat(frontend): contract types + REST client (MSW-tested)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/api/client.ts

@@ -0,0 +1,12 @@
+const BASE = import.meta.env.VITE_BACKEND_URL ?? "http://localhost:8787";
+
+export async function apiFetch<T>(path: string, init?: RequestInit): Promise<T> {
+  const res = await fetch(`${BASE}${path}`, {
+    headers: { "content-type": "application/json" },
+    ...init,
+  });
+  if (!res.ok) throw new Error(`${res.status} ${await res.text().catch(() => "")}`);
+  return res.status === 204 ? (undefined as T) : ((await res.json()) as T);
+}
+
+export { BASE };