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>

vitest.config.ts

@@ -0,0 +1,33 @@
+import { defineConfig } from 'vitest/config';
+import react from '@vitejs/plugin-react';
+import path from 'path';
+
+export default defineConfig({
+  plugins: [react()],
+  test: {
+    globals: true,
+    environment: 'jsdom',
+    setupFiles: ['./tests/setup.ts'],
+    include: ['tests/**/*.test.ts', 'tests/**/*.test.tsx'],
+    coverage: {
+      provider: 'v8',
+      reporter: ['text', 'json', 'html'],
+      reportsDirectory: './coverage',
+      include: ['src/**/*.ts', 'desktop/src/**/*.ts', 'desktop/src/**/*.tsx'],
+      exclude: [
+        'src/**/*.d.ts',
+        'desktop/src/**/*.d.ts',
+        'tests/**',
+        '**/*.test.ts',
+        '**/*.test.tsx',
+        '**/node_modules/**',
+      ],
+    },
+  },
+  resolve: {
+    alias: {
+      '@': path.resolve(__dirname, './desktop/src'),
+      '@lib': path.resolve(__dirname, './desktop/src/lib'),
+    },
+  },
+});