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/i18n/locales/zh.ts

@@ -944,6 +944,7 @@ export default {
     noPricing: '未配置定价',
     columns: {
       name: '渠道名',
+      platform: '平台',
       groups: '我可访问的分组',
       supportedModels: '支持模型'
     },