fix(channel): 渠道表单校验增强 — 空模型定价报错 + 必填标记

- 保存时校验：定价条目有但模型列表为空时报错并跳转到对应平台 tab，
  不再静默跳过导致数据丢失
- 保存时校验：启用的平台必须至少选择一个分组
- 分组关联标签增加红色 * 必填标记

frontend/src/views/admin/ChannelsView.vue

@@ -261,7 +261,7 @@
             <!-- Groups -->
             <div>
               <label class="input-label text-xs">
-                {{ t('admin.channels.form.groups', 'Associated Groups') }}
+                {{ t('admin.channels.form.groups', 'Associated Groups') }} <span class="text-red-500">*</span>
                 <span v-if="section.group_ids.length > 0" class="ml-1 font-normal text-gray-400">
                   ({{ t('admin.channels.form.selectedCount', { count: section.group_ids.length }, `已选 ${section.group_ids.length} 个`) }})
                 </span>
@@ -859,6 +859,24 @@ async function handleSubmit() {
     return
   }
 
+  // Check for pricing entries with empty models (would be silently skipped)
+  for (const section of form.platforms.filter(s => s.enabled)) {
+    if (section.group_ids.length === 0) {
+      const platformLabel = t('admin.groups.platforms.' + section.platform, section.platform)
+      appStore.showError(t('admin.channels.noGroupsSelected', { platform: platformLabel }, `${platformLabel} 平台未选择分组，请至少选择一个分组或禁用该平台`))
+      activeTab.value = section.platform
+      return
+    }
+    for (const entry of section.model_pricing) {
+      if (entry.models.length === 0) {
+        const platformLabel = t('admin.groups.platforms.' + section.platform, section.platform)
+        appStore.showError(t('admin.channels.emptyModelsInPricing', { platform: platformLabel }, `${platformLabel} 平台下有定价条目未添加模型，请添加模型或删除该条目`))
+        activeTab.value = section.platform
+        return
+      }
+    }
+  }
+
   // Check duplicate models across all enabled platform sections
   const allModels = form.platforms.filter(s => s.enabled).flatMap(s => s.model_pricing.flatMap(e => e.models.map(m => m.toLowerCase())))
   const duplicates = allModels.filter((m, i) => allModels.indexOf(m) !== i)