feat(groups): add rate multipliers management modal

Add a dedicated modal in group management for viewing, adding, editing,
and deleting per-user rate multipliers within a group.

Backend:
- GET /admin/groups/:id/rate-multipliers - list entries with user details
- PUT /admin/groups/:id/rate-multipliers - batch sync (full replace)
- DELETE /admin/groups/:id/rate-multipliers - clear all entries
- Repository: GetByGroupID, SyncGroupRateMultipliers methods on
  user_group_rate_multipliers table (same table as user-side rates)

Frontend:
- New GroupRateMultipliersModal component with:
  - User search and add with email autocomplete
  - Editable rate column with local edit mode (cancel/save)
  - Batch adjust: multiply all rates by a factor
  - Clear all (local operation, requires save to persist)
  - Pagination (10/20/50 per page)
  - Platform icon with brand colors in group info bar
  - Unsaved changes indicator with revert option
- Unit tests for all three backend endpoints

frontend/src/i18n/locales/zh.ts

@@ -1428,7 +1428,11 @@ export default {
         accounts: '账号数',
         status: '状态',
         actions: '操作',
-        billingType: '计费类型'
+        billingType: '计费类型',
+        userName: '用户名',
+        userEmail: '邮箱',
+        userNotes: '备注',
+        userStatus: '状态'
       },
       form: {
         name: '名称',
@@ -1510,6 +1514,26 @@ export default {
       failedToCreate: '创建分组失败',
       failedToUpdate: '更新分组失败',
       nameRequired: '请输入分组名称',
+      rateMultipliers: '专属倍率',
+      rateMultipliersTitle: '分组专属倍率管理',
+      addUserRate: '添加用户专属倍率',
+      searchUserPlaceholder: '搜索用户邮箱...',
+      noRateMultipliers: '暂无用户设置了专属倍率',
+      rateUpdated: '专属倍率已更新',
+      rateDeleted: '专属倍率已删除',
+      rateAdded: '专属倍率已添加',
+      clearAll: '全部清空',
+      confirmClearAll: '确定要清空该分组所有用户的专属倍率设置吗？此操作不可撤销。',
+      rateCleared: '已清空所有专属倍率',
+      batchAdjust: '批量调整倍率',
+      multiplierFactor: '乘数',
+      applyMultiplier: '应用',
+      rateAdjusted: '倍率已批量调整',
+      rateSaved: '专属倍率已保存',
+      finalRate: '最终倍率',
+      unsavedChanges: '有未保存的修改',
+      revertChanges: '撤销修改',
+      userInfo: '用户信息',
       subscription: {
         title: '订阅设置',
         type: '计费类型',