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

backend/internal/service/user_group_rate.go

@@ -2,6 +2,22 @@ package service
 
 import "context"
 
+// UserGroupRateEntry 分组下用户专属倍率条目
+type UserGroupRateEntry struct {
+	UserID         int64   `json:"user_id"`
+	UserName       string  `json:"user_name"`
+	UserEmail      string  `json:"user_email"`
+	UserNotes      string  `json:"user_notes"`
+	UserStatus     string  `json:"user_status"`
+	RateMultiplier float64 `json:"rate_multiplier"`
+}
+
+// GroupRateMultiplierInput 批量设置分组倍率的输入条目
+type GroupRateMultiplierInput struct {
+	UserID         int64   `json:"user_id"`
+	RateMultiplier float64 `json:"rate_multiplier"`
+}
+
 // UserGroupRateRepository 用户专属分组倍率仓储接口
 // 允许管理员为特定用户设置分组的专属计费倍率，覆盖分组默认倍率
 type UserGroupRateRepository interface {
@@ -13,10 +29,16 @@ type UserGroupRateRepository interface {
 	// 如果未设置专属倍率，返回 nil
 	GetByUserAndGroup(ctx context.Context, userID, groupID int64) (*float64, error)
 
+	// GetByGroupID 获取指定分组下所有用户的专属倍率
+	GetByGroupID(ctx context.Context, groupID int64) ([]UserGroupRateEntry, error)
+
 	// SyncUserGroupRates 同步用户的分组专属倍率
 	// rates: map[groupID]*rateMultiplier，nil 表示删除该分组的专属倍率
 	SyncUserGroupRates(ctx context.Context, userID int64, rates map[int64]*float64) error
 
+	// SyncGroupRateMultipliers 批量同步分组的用户专属倍率（替换整组数据）
+	SyncGroupRateMultipliers(ctx context.Context, groupID int64, entries []GroupRateMultiplierInput) error
+
 	// DeleteByGroupID 删除指定分组的所有用户专属倍率（分组删除时调用）
 	DeleteByGroupID(ctx context.Context, groupID int64) error