feat(channels): aggregate by channel with platform sections + rowspan table

Switch the user-facing 'Available Channels' view from "one row per
platform" to "one channel row-group with N platform sections".

Backend: userAvailableChannel now holds Platforms []section instead
of being exploded. buildPlatformSections replaces
explodeChannelByPlatform with the same per-platform grouping logic.

Frontend: drop the DataTable wrapper for this view and write a
four-column grid table (渠道名 / 平台 / 分组 / 支持模型) where the
channel name only renders on the first platform row of each channel —
visual rowspan without hacking DataTable.

- api/channels.ts: UserChannelPlatformSection + platforms[]
- AvailableChannelsTable: rewritten as native grid (header + per-
  channel section with hover row highlight)
- AvailableChannelsView: search now filters platforms sub-array;
  channel-name / description hits still keep the whole channel
- i18n: add availableChannels.columns.platform (zh/en)

frontend/src/api/channels.ts

@@ -40,18 +40,23 @@ export interface UserSupportedModel {
   pricing: UserSupportedModelPricing | null
 }
 
-export interface UserAvailableChannel {
-  name: string
-  description: string
-  /**
-   * 所属平台（anthropic / openai / antigravity / gemini ...）。后端按平台把一个渠道
-   * 摊开成多条记录，因此此字段决定整行的配色与图标。
-   */
+/**
+ * 渠道下单个平台的子视图：用户可访问的分组 + 该平台支持的模型。
+ * 后端把一个渠道按平台聚合成 sections，前端可以把渠道名作为 row-group
+ * 一次渲染，后面按 sections 顺序用 rowspan 铺开。
+ */
+export interface UserChannelPlatformSection {
   platform: string
   groups: UserAvailableGroup[]
   supported_models: UserSupportedModel[]
 }
 
+export interface UserAvailableChannel {
+  name: string
+  description: string
+  platforms: UserChannelPlatformSection[]
+}
+
 /** 列出当前用户可见的「可用渠道」（与 /groups/available 保持一致，返回平数组）。 */
 export async function getAvailable(options?: { signal?: AbortSignal }): Promise<UserAvailableChannel[]> {
   const { data } = await apiClient.get<UserAvailableChannel[]>('/channels/available', {