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/components/admin/user/GroupReplaceModal.vue

@@ -0,0 +1,131 @@
+<template>
+  <BaseDialog :show="show" :title="t('admin.users.replaceGroupTitle')" width="narrow" @close="$emit('close')">
+    <div v-if="oldGroup" class="space-y-4">
+      <!-- 提示信息 -->
+      <p class="text-sm text-gray-600 dark:text-gray-400">
+        {{ t('admin.users.replaceGroupHint', { old: oldGroup.name }) }}
+      </p>
+
+      <!-- 当前分组 -->
+      <div class="rounded-lg border border-gray-200 bg-gray-50 p-3 dark:border-dark-600 dark:bg-dark-800">
+        <div class="flex items-center gap-2">
+          <Icon name="shield" size="sm" class="text-purple-500" />
+          <span class="font-medium text-gray-900 dark:text-white">{{ oldGroup.name }}</span>
+          <Icon name="arrowRight" size="sm" class="ml-auto text-gray-400" />
+          <span v-if="selectedGroupId" class="font-medium text-primary-600 dark:text-primary-400">
+            {{ availableGroups.find(g => g.id === selectedGroupId)?.name }}
+          </span>
+          <span v-else class="text-sm text-gray-400">?</span>
+        </div>
+      </div>
+
+      <!-- 可选分组列表 -->
+      <div v-if="availableGroups.length > 0" class="max-h-64 space-y-2 overflow-y-auto">
+        <label
+          v-for="group in availableGroups"
+          :key="group.id"
+          class="flex cursor-pointer items-center gap-3 rounded-lg border-2 p-3 transition-all"
+          :class="selectedGroupId === group.id
+            ? 'border-primary-400 bg-primary-50/50 dark:border-primary-500 dark:bg-primary-900/20'
+            : 'border-gray-200 hover:border-gray-300 dark:border-dark-600 dark:hover:border-dark-500'"
+        >
+          <input
+            type="radio"
+            :value="group.id"
+            v-model="selectedGroupId"
+            class="sr-only"
+          />
+          <div
+            class="flex h-5 w-5 items-center justify-center rounded-full border-2 transition-all"
+            :class="selectedGroupId === group.id
+              ? 'border-primary-500 bg-primary-500'
+              : 'border-gray-300 dark:border-dark-500'"
+          >
+            <div v-if="selectedGroupId === group.id" class="h-2 w-2 rounded-full bg-white"></div>
+          </div>
+          <div class="flex-1">
+            <span class="font-medium text-gray-900 dark:text-white">{{ group.name }}</span>
+            <span class="ml-2 text-xs text-gray-400">{{ group.platform }}</span>
+          </div>
+        </label>
+      </div>
+
+      <!-- 无可选分组 -->
+      <div v-else class="py-6 text-center text-sm text-gray-400">
+        {{ t('admin.users.noOtherGroups') }}
+      </div>
+    </div>
+
+    <template #footer>
+      <div class="flex justify-end gap-3">
+        <button @click="$emit('close')" class="btn btn-secondary px-5">{{ t('common.cancel') }}</button>
+        <button
+          @click="handleReplace"
+          :disabled="!selectedGroupId || submitting"
+          class="btn btn-primary px-6"
+        >
+          <svg v-if="submitting" class="-ml-1 mr-2 h-4 w-4 animate-spin" fill="none" viewBox="0 0 24 24">
+            <circle class="opacity-25" cx="12" cy="12" r="10" stroke="currentColor" stroke-width="4"></circle>
+            <path class="opacity-75" fill="currentColor" d="M4 12a8 8 0 018-8V0C5.373 0 0 5.373 0 12h4zm2 5.291A7.962 7.962 0 014 12H0c0 3.042 1.135 5.824 3 7.938l3-2.647z"></path>
+          </svg>
+          {{ submitting ? t('common.saving') : t('admin.users.replaceGroupConfirm') }}
+        </button>
+      </div>
+    </template>
+  </BaseDialog>
+</template>
+
+<script setup lang="ts">
+import { ref, watch, computed } from 'vue'
+import { useI18n } from 'vue-i18n'
+import { useAppStore } from '@/stores/app'
+import { adminAPI } from '@/api/admin'
+import type { AdminUser, AdminGroup } from '@/types'
+import BaseDialog from '@/components/common/BaseDialog.vue'
+import Icon from '@/components/icons/Icon.vue'
+
+interface Props {
+  show: boolean
+  user: AdminUser | null
+  oldGroup: { id: number; name: string } | null
+  allGroups: AdminGroup[]
+}
+
+const props = defineProps<Props>()
+const emit = defineEmits(['close', 'success'])
+const { t } = useI18n()
+const appStore = useAppStore()
+
+const selectedGroupId = ref<number | null>(null)
+const submitting = ref(false)
+
+// 可选的专属标准分组（排除当前 oldGroup）
+const availableGroups = computed(() => {
+  if (!props.oldGroup) return []
+  return props.allGroups.filter(
+    g => g.status === 'active' && g.is_exclusive && g.subscription_type === 'standard' && g.id !== props.oldGroup!.id
+  )
+})
+
+watch(() => props.show, (v) => {
+  if (v) {
+    selectedGroupId.value = null
+  }
+})
+
+const handleReplace = async () => {
+  if (!props.user || !props.oldGroup || !selectedGroupId.value) return
+  submitting.value = true
+
+  try {
+    const result = await adminAPI.users.replaceGroup(props.user.id, props.oldGroup.id, selectedGroupId.value)
+    appStore.showSuccess(t('admin.users.replaceGroupSuccess', { count: result.migrated_keys }))
+    emit('success')
+    emit('close')
+  } catch (error) {
+    console.error('Failed to replace group:', error)
+  } finally {
+    submitting.value = false
+  }
+}
+</script>