feat: 数据格式改为按钮组选择

- 移除 Select 下拉框
- 改为 JSON/CSV 按钮组
- 选中状态与生成数量按钮样式统一

feat: 优化空值率设置

- 增加 75% 预设选项
- 设置 100% 时自动设为必填
- 限制输入范围 0-100

fix: 空值率预设改为纯数字显示

- 5%/20%/50%/75% 统一样式

fix: 移除空值率低/中/高 i18n 翻译键

fix: 精简生成数量预设选项

- 移除 10、500
- 保留 50、100、1000、5000、10000

public/_locales/zh_CN/messages.json

@@ -971,18 +971,6 @@
     "message": "空值率",
     "description": "Translation key: testDataGenerator_nullRate"
   },
-  "testDataGenerator_nullRateLow": {
-    "message": "低 (5%)",
-    "description": "Translation key: testDataGenerator_nullRateLow"
-  },
-  "testDataGenerator_nullRateMedium": {
-    "message": "中 (20%)",
-    "description": "Translation key: testDataGenerator_nullRateMedium"
-  },
-  "testDataGenerator_nullRateHigh": {
-    "message": "高 (50%)",
-    "description": "Translation key: testDataGenerator_nullRateHigh"
-  },
   "testDataGenerator_uniqueConstraint": {
     "message": "唯一性约束",
     "description": "Translation key: testDataGenerator_uniqueConstraint"