fix: mock fetch in test setup to suppress URL parse errors in jsdom

Also set jsdom base URL to http://localhost:3000 in vitest config.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/setupTests.ts

@@ -1 +1,14 @@
 import "@testing-library/jest-dom";
+import { beforeEach, vi } from "vitest";
+
+// Provide a default fetch mock so components that call API endpoints on mount
+// don't throw URL-parse errors in the jsdom test environment.  Tests that need
+// specific responses should mock the relevant `api.*` method as usual.
+beforeEach(() => {
+	vi.stubGlobal(
+		"fetch",
+		vi.fn(() =>
+			Promise.resolve(new Response(JSON.stringify({}), { status: 200 })),
+		),
+	);
+});