feat(channel): 模型映射 + 分组搜索 + 卡片折叠 + 冲突校验

- 渠道模型映射：新增 model_mapping JSONB 字段，在账号映射之前执行
- 分组选择：添加搜索过滤 + 平台图标
- 定价卡片：支持折叠/展开，已有数据默认折叠
- 模型冲突校验：前后端均禁止同一渠道内重复模型
- 迁移 083: channels 表添加 model_mapping 列

frontend/src/i18n/locales/en.ts

@@ -1743,6 +1743,7 @@ export default {
       updateError: 'Failed to update channel',
       deleteError: 'Failed to delete channel',
       nameRequired: 'Please enter a channel name',
+      duplicateModels: 'Model "{0}" appears in multiple pricing entries',
       deleteConfirm: 'Are you sure you want to delete channel "{name}"? This cannot be undone.',
       columns: {
         name: 'Name',