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/views/admin/GroupsView.vue

@@ -181,6 +181,13 @@
                 <Icon name="edit" size="sm" />
                 <span class="text-xs">{{ t('common.edit') }}</span>
               </button>
+              <button
+                @click="handleRateMultipliers(row)"
+                class="flex flex-col items-center gap-0.5 rounded-lg p-1.5 text-gray-500 transition-colors hover:bg-gray-100 hover:text-purple-600 dark:hover:bg-dark-700 dark:hover:text-purple-400"
+              >
+                <Icon name="dollar" size="sm" />
+                <span class="text-xs">{{ t('admin.groups.rateMultipliers') }}</span>
+              </button>
               <button
                 @click="handleDelete(row)"
                 class="flex flex-col items-center gap-0.5 rounded-lg p-1.5 text-gray-500 transition-colors hover:bg-red-50 hover:text-red-600 dark:hover:bg-red-900/20 dark:hover:text-red-400"
@@ -1775,6 +1782,14 @@
         </div>
       </template>
     </BaseDialog>
+
+    <!-- Group Rate Multipliers Modal -->
+    <GroupRateMultipliersModal
+      :show="showRateMultipliersModal"
+      :group="rateMultipliersGroup"
+      @close="showRateMultipliersModal = false"
+      @success="loadGroups"
+    />
   </AppLayout>
 </template>
 
@@ -1796,6 +1811,7 @@ import EmptyState from '@/components/common/EmptyState.vue'
 import Select from '@/components/common/Select.vue'
 import PlatformIcon from '@/components/common/PlatformIcon.vue'
 import Icon from '@/components/icons/Icon.vue'
+import GroupRateMultipliersModal from '@/components/admin/group/GroupRateMultipliersModal.vue'
 import { VueDraggable } from 'vue-draggable-plus'
 import { createStableObjectKeyResolver } from '@/utils/stableObjectKey'
 import { useKeyedDebouncedSearch } from '@/composables/useKeyedDebouncedSearch'
@@ -1970,6 +1986,8 @@ const submitting = ref(false)
 const sortSubmitting = ref(false)
 const editingGroup = ref<AdminGroup | null>(null)
 const deletingGroup = ref<AdminGroup | null>(null)
+const showRateMultipliersModal = ref(false)
+const rateMultipliersGroup = ref<AdminGroup | null>(null)
 const sortableGroups = ref<AdminGroup[]>([])
 
 const createForm = reactive({
@@ -2459,6 +2477,11 @@ const handleUpdateGroup = async () => {
   }
 }
 
+const handleRateMultipliers = (group: AdminGroup) => {
+  rateMultipliersGroup.value = group
+  showRateMultipliersModal.value = true
+}
+
 const handleDelete = (group: AdminGroup) => {
   deletingGroup.value = group
   showDeleteDialog.value = true