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/src/hooks/useThemeMode.test.ts

@@ -0,0 +1,15 @@
+import { describe, it, expect } from 'vitest'
+import { renderHook } from '@testing-library/react'
+import { useThemeMode } from './useThemeMode'
+
+describe('useThemeMode', () => {
+  it('should return false when no ConfigProvider is present (light default)', () => {
+    const { result } = renderHook(() => useThemeMode())
+    expect(result.current).toBe(false)
+  })
+
+  it('should return a boolean value', () => {
+    const { result } = renderHook(() => useThemeMode())
+    expect(typeof result.current).toBe('boolean')
+  })
+})