feat: 添加简单模式功能

新增简单模式设置，适合个人使用场景：
- 隐藏多用户管理相关菜单（用户管理、兑换码等）
- 自动关闭用户注册功能
- 管理员并发数自动设为无限制(99999)
- 侧边栏根据模式动态调整菜单项

同时优化分组页面的"专属分组"功能，添加帮助提示说明使用场景

frontend/src/i18n/locales/en.ts

@@ -676,14 +676,21 @@ export default {
         description: 'Description',
         platform: 'Platform',
         rateMultiplier: 'Rate Multiplier',
-        status: 'Status'
+        status: 'Status',
+        exclusive: 'Exclusive Group'
       },
       enterGroupName: 'Enter group name',
       optionalDescription: 'Optional description',
       platformHint: 'Select the platform this group is associated with',
       platformNotEditable: 'Platform cannot be changed after creation',
       rateMultiplierHint: 'Cost multiplier for this group (e.g., 1.5 = 150% of base cost)',
-      exclusiveHint: 'Exclusive (requires explicit user access)',
+      exclusiveHint: 'Exclusive group, manually assign to specific users',
+      exclusiveTooltip: {
+        title: 'What is an exclusive group?',
+        description: 'When enabled, users cannot see this group when creating API Keys. Only after an admin manually assigns a user to this group can they use it.',
+        example: 'Use case:',
+        exampleContent: 'Public group rate is 0.8. Create an exclusive group with 0.7 rate, manually assign VIP users to give them better pricing.'
+      },
       noGroupsYet: 'No groups yet',
       createFirstGroup: 'Create your first group to organize API keys.',
       creating: 'Creating...',
@@ -1400,6 +1407,16 @@ 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',

frontend/src/i18n/locales/zh.ts

@@ -733,14 +733,15 @@ export default {
         platform: '平台',
         rateMultiplier: '费率倍数',
         status: '状态',
+        exclusive: '专属分组',
         nameLabel: '分组名称',
         namePlaceholder: '请输入分组名称',
         descriptionLabel: '描述',
         descriptionPlaceholder: '请输入描述（可选）',
         rateMultiplierLabel: '费率倍数',
         rateMultiplierHint: '1.0 = 标准费率，0.5 = 半价，2.0 = 双倍',
-        exclusiveLabel: '独占模式',
-        exclusiveHint: '启用后，此分组的用户将独占使用分配的账号',
+        exclusiveLabel: '专属分组',
+        exclusiveHint: '专属分组，可以手动指定给用户',
         platformLabel: '平台限制',
         platformPlaceholder: '选择平台（留空则不限制）',
         accountsLabel: '指定账号',
@@ -753,8 +754,14 @@ export default {
         yes: '是',
         no: '否'
       },
-      exclusive: '独占',
-      exclusiveHint: '启用后，此分组的用户将独占使用分配的账号',
+      exclusive: '专属',
+      exclusiveHint: '专属分组，可以手动指定给特定用户',
+      exclusiveTooltip: {
+        title: '什么是专属分组？',
+        description: '开启后，用户在创建 API Key 时将无法看到此分组。只有管理员手动将用户分配到此分组后，用户才能使用。',
+        example: '使用场景：',
+        exampleContent: '公开分组费率 0.8，您可以创建一个费率 0.7 的专属分组，手动分配给 VIP 用户，让他们享受更优惠的价格。'
+      },
       rateMultiplierHint: '1.0 = 标准费率，0.5 = 半价，2.0 = 双倍',
       platforms: {
         all: '全部平台',
@@ -773,8 +780,8 @@ export default {
       allPlatforms: '全部平台',
       allStatus: '全部状态',
       allGroups: '全部分组',
-      exclusiveFilter: '独占',
-      nonExclusive: '非独占',
+      exclusiveFilter: '专属',
+      nonExclusive: '公开',
       public: '公开',
       rateAndAccounts: '{rate}x 费率 · {count} 个账号',
       accountsCount: '{count} 个账号',
@@ -1597,6 +1604,16 @@ export default {
         securityWarning: '警告：此密钥拥有完整的管理员权限，请妥善保管。',
         usage: '使用方法：在请求头中添加 x-api-key: <your-admin-api-key>'
       },
+      usageMode: {
+        title: '使用模式',
+        description: '切换简单模式以简化界面',
+        simpleMode: '简单模式',
+        simpleModeHint: '隐藏多用户管理功能，适合个人使用',
+        simpleModeWarning: '启用后将隐藏用户管理、兑换码管理等菜单，关闭用户注册功能，并将管理员并发数设为无限制',
+        confirmTitle: '确认切换使用模式',
+        confirmEnableMessage: '确定要启用简单模式吗？启用后将隐藏多用户管理相关菜单、关闭用户注册功能，并将管理员并发数设为 99999（无限制）。',
+        confirmDisableMessage: '确定要关闭简单模式吗？关闭后将恢复显示所有管理菜单。'
+      },
       saveSettings: '保存设置',
       saving: '保存中...',
       settingsSaved: '设置保存成功',