test: configure Vitest testing framework

- Add vitest, @testing-library/react, @vitest/ui, jsdom dependencies
- Create vitest.config.ts with jsdom environment and coverage settings
- Add tests/setup.ts with localStorage mock and crypto mock
- Add tests/gateway/ws-client.test.ts for GatewayClient unit tests

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

tests/setup.ts

@@ -0,0 +1,50 @@
+/**
+ * Test setup file for Vitest
+ * Configure global test environment
+ */
+
+import { beforeAll, beforeEach } from 'vitest';
+
+// Mock localStorage
+const localStorageMock = (() => {
+  let store: Record<string, string> = {};
+  return {
+    getItem: (key: string) => store[key] || null,
+    setItem: (key: string, value: string) => {
+      store[key] = value.toString();
+    },
+    removeItem: (key: string) => {
+      delete store[key];
+    },
+    clear: () => {
+      store = {};
+    },
+    get length() {
+      return Object.keys(store).length;
+    },
+    key: (index: number) => {
+      return Object.keys(store)[index] || null;
+    },
+  };
+})();
+
+// Setup global mocks
+beforeAll(() => {
+  // Mock crypto.randomUUID if not available
+  if (!globalThis.crypto) {
+    globalThis.crypto = {
+      randomUUID: () => 'test-uuid-1234',
+      subtle: {
+        digest: async () => new ArrayBuffer(16),
+      },
+    } as any;
+  }
+});
+
+// Setup before each test
+beforeEach(() => {
+  localStorageMock.clear();
+});
+
+// Export for use in tests
+export { localStorageMock };