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

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

frontend/src/test/msw.ts

@@ -0,0 +1,2 @@
+import { setupServer } from "msw/node";
+export const server = setupServer();

frontend/src/test/setup.ts

@@ -1 +1,6 @@
 import "@testing-library/jest-dom/vitest";
+import { server } from "./msw";
+
+beforeAll(() => server.listen());
+afterEach(() => server.resetHandlers());
+afterAll(() => server.close());