refactor: 移除旧版数据库配置的简易模式实现

移除与 PR #66 冲突的旧版简易模式实现（commit 7d4b7de）。
新版简易模式通过 run_mode 配置文件/环境变量控制，无需数据库设置。

后端变更：
- 移除 SettingKeySimpleMode 常量
- 移除 SystemSettings/PublicSettings 中的 SimpleMode 字段
- 移除 setting_handler 中的简易模式切换逻辑
- 移除 userService 依赖（不再需要自动设置管理员并发数）

前端变更：
- 移除 appStore.simpleMode 状态
- 移除设置页面的"使用模式"设置区块
- 移除 GroupsView 中的简易模式相关逻辑
- 移除相关国际化文案

frontend/src/i18n/locales/en.ts

@@ -1413,16 +1413,6 @@ export default {
         securityWarning: 'Warning: This key provides full admin access. Keep it secure.',
         usage: 'Usage: Add to request header - x-api-key: <your-admin-api-key>'
       },
-      usageMode: {
-        title: 'Usage Mode',
-        description: 'Toggle simple mode for a simplified interface',
-        simpleMode: 'Simple Mode',
-        simpleModeHint: 'Hide multi-user management features, suitable for personal use',
-        simpleModeWarning: 'When enabled, user management and redeem code menus will be hidden, registration will be disabled, and admin concurrency will be set to unlimited',
-        confirmTitle: 'Confirm Mode Change',
-        confirmEnableMessage: 'Are you sure you want to enable Simple Mode? This will hide multi-user management menus, disable registration, and set admin concurrency to 99999 (unlimited).',
-        confirmDisableMessage: 'Are you sure you want to disable Simple Mode? This will restore all management menus.'
-      },
       saveSettings: 'Save Settings',
       saving: 'Saving...',
       settingsSaved: 'Settings saved successfully',