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)
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