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',

frontend/src/i18n/locales/zh.ts

@@ -1314,6 +1314,9 @@ export default {
       roleFilter: '角色筛选',
       allRoles: '全部角色',
       allStatus: '全部状态',
+      allGroups: '全部分组',
+      searchGroups: '搜索分组...',
+      fuzzySearch: '模糊搜索',
       statusFilter: '状态筛选',
       allStatuses: '全部状态',
       admin: '管理员',
@@ -1340,6 +1343,7 @@ export default {
         username: '用户名',
         notes: '备注',
         role: '角色',
+        groups: '分组',
         subscriptions: '订阅分组',
         balance: '余额',
         usage: '用量',
@@ -1351,6 +1355,9 @@ export default {
       today: '今日',
       total: '近30天',
       noSubscription: '暂无订阅',
+      publicGroupCount: '+{count} 公开',
+      exclusiveLabel: '专属',
+      publicLabel: '公开',
       daysRemaining: '{days}天',
       expired: '已过期',
       disable: '禁用',
@@ -1442,6 +1449,14 @@ export default {
       useDefaultRate: '使用默认',
       customRatePlaceholder: '留空使用默认',
       groupConfigUpdated: '分组配置更新成功',
+      replaceGroup: '替换分组',
+      clickToReplace: '点击替换分组',
+      replaceGroupTitle: '替换专属分组',
+      replaceGroupHint: '选择新分组替换「{old}」，将自动迁移绑定的 Key 并更新分组权限',
+      replaceGroupConfirm: '确认替换',
+      replaceGroupSuccess: '分组替换成功，已迁移 {count} 个 Key',
+      selectNewGroup: '请选择目标分组',
+      noOtherGroups: '没有其他可用的专属分组',
       deposit: '充值',
       withdraw: '退款',
       depositAmount: '充值金额',