Add MSW infrastructure: handlers, fixtures, request-log, and Vitest setup

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

tests/setup-msw.js

@@ -0,0 +1,15 @@
+/**
+ * Vitest global setup for MSW.
+ * Starts the MSW server before all tests, resets handlers
+ * and request log after each test, closes after all tests.
+ */
+import { beforeAll, afterEach, afterAll } from "vitest";
+import { server } from "@/mocks/node.js";
+import { clearRequests } from "@/mocks/request-log.js";
+
+beforeAll(() => server.listen({ onUnhandledRequest: "warn" }));
+afterEach(() => {
+  server.resetHandlers();
+  clearRequests();
+});
+afterAll(() => server.close());