feat(admin): 用户管理新增分组列、分组筛选与专属分组一键替换

- 新增分组列：展示用户的专属/公开分组，支持 hover 查看详情
- 新增分组筛选：下拉选择或模糊搜索分组名过滤用户
- 专属分组替换：点击专属分组弹出操作菜单，选择目标分组后
  自动授予新分组权限、迁移绑定的 Key、移除旧分组权限
- 后端新增 POST /admin/users/:id/replace-group 端点，事务内
  完成分组替换并失效认证缓存

frontend/src/i18n/locales/en.ts

@@ -1289,6 +1289,9 @@ export default {
       searchUsers: 'Search by email, username, notes, or API key...',
       allRoles: 'All Roles',
       allStatus: 'All Status',
+      allGroups: 'All Groups',
+      searchGroups: 'Search groups...',
+      fuzzySearch: 'Fuzzy search',
       admin: 'Admin',
       user: 'User',
       disabled: 'Disabled',
@@ -1313,6 +1316,7 @@ export default {
         username: 'Username',
         notes: 'Notes',
         role: 'Role',
+        groups: 'Groups',
         subscriptions: 'Subscriptions',
         balance: 'Balance',
         usage: 'Usage',
@@ -1324,6 +1328,9 @@ export default {
       today: 'Today',
       total: 'Last 30d',
       noSubscription: 'No subscription',
+      publicGroupCount: '+{count} public',
+      exclusiveLabel: 'exclusive',
+      publicLabel: 'public',
       daysRemaining: '{days}d',
       expired: 'Expired',
       disable: 'Disable',
@@ -1379,6 +1386,14 @@ export default {
       useDefaultRate: 'Use Default',
       customRatePlaceholder: 'Leave empty for default',
       groupConfigUpdated: 'Group configuration updated successfully',
+      replaceGroup: 'Replace Group',
+      clickToReplace: 'Click to replace',
+      replaceGroupTitle: 'Replace Exclusive Group',
+      replaceGroupHint: 'Select a new group to replace "{old}". Keys will be migrated and permissions updated automatically.',
+      replaceGroupConfirm: 'Confirm Replace',
+      replaceGroupSuccess: 'Group replaced successfully, {count} key(s) migrated',
+      selectNewGroup: 'Select target group',
+      noOtherGroups: 'No other exclusive groups available',
       deposit: 'Deposit',
       withdraw: 'Withdraw',
       depositAmount: 'Deposit Amount',