test(web): 添加 vitest 单元测试基础设施和初始测试用例

- 安装 vitest + @testing-library/react + @testing-library/jest-dom + jsdom
- 创建 vitest.config.ts (jsdom 环境, 全局 API, e2e 目录排除)
- 创建 test/setup.ts (@testing-library/jest-dom 匹配器)
- 添加 29 个测试用例: health 常量 (14), useThemeMode hook (2), StatusTag 组件 (13)

apps/web/vitest.config.ts

@@ -0,0 +1,18 @@
+import { defineConfig } from 'vitest/config'
+import react from '@vitejs/plugin-react'
+import path from 'path'
+
+export default defineConfig({
+  plugins: [react()],
+  test: {
+    environment: 'jsdom',
+    globals: true,
+    setupFiles: ['./src/test/setup.ts'],
+    exclude: ['e2e/**', 'node_modules/**'],
+  },
+  resolve: {
+    alias: {
+      '@': path.resolve(__dirname, './src'),
+    },
+  },
+})