test: add Vitest configuration and setup

- Add vitest.config.ts with jsdom environment and path aliases
- Add tests/setup.ts with mocks for Tauri API, crypto, and localStorage
- Add test:coverage script to package.json

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

desktop/tests/setup.ts

@@ -0,0 +1,76 @@
+import '@testing-library/jest-dom';
+import { vi } from 'vitest';
+import { webcrypto } from 'node:crypto';
+
+// Polyfill Web Crypto API for Node.js test environment
+Object.defineProperty(global, 'crypto', {
+  value: webcrypto,
+  configurable: true,
+});
+
+// Mock Tauri API
+vi.mock('@tauri-apps/api/core', () => ({
+  invoke: vi.fn(),
+}));
+
+// Mock Tauri runtime check
+vi.mock('../src/lib/tauri-gateway', () => ({
+  isTauriRuntime: () => false,
+  getGatewayClient: vi.fn(),
+  startLocalGateway: vi.fn(),
+  stopLocalGateway: vi.fn(),
+  getLocalGatewayStatus: vi.fn(),
+  getLocalGatewayAuth: vi.fn(),
+  prepareLocalGatewayForTauri: vi.fn(),
+  approveLocalGatewayDevicePairing: vi.fn(),
+  getOpenFangProcessList: vi.fn(),
+  getOpenFangProcessLogs: vi.fn(),
+}));
+
+// Mock localStorage with export for test access
+export const localStorageMock = (() => {
+  let store: Record<string, string> = {};
+  return {
+    getItem: (key: string) => store[key] || null,
+    setItem: (key: string, value: string) => {
+      store[key] = value;
+    },
+    removeItem: (key: string) => {
+      delete store[key];
+    },
+    clear: () => {
+      store = {};
+    },
+    get length() {
+      return Object.keys(store).length;
+    },
+    key: (index: number) => {
+      const keys = Object.keys(store);
+      return keys[index] || null;
+    },
+  };
+})();
+
+Object.defineProperty(global, 'localStorage', {
+  value: localStorageMock,
+  configurable: true,
+});
+
+// Mock crypto.subtle for tests (already set via webcrypto above, but ensure subtle mock works)
+const subtleMock = {
+  encrypt: vi.fn(),
+  decrypt: vi.fn(),
+  generateKey: vi.fn(),
+  deriveKey: vi.fn(),
+  importKey: vi.fn(),
+  exportKey: vi.fn(),
+  digest: vi.fn(),
+  sign: vi.fn(),
+  verify: vi.fn(),
+};
+
+// Override subtle if needed for specific test scenarios
+Object.defineProperty(global.crypto, 'subtle', {
+  value: subtleMock,
+  configurable: true,
+});