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/views/user/AvailableChannelsView.vue

@@ -34,7 +34,7 @@
 
       <template #table>
         <AvailableChannelsTable
-          :columns="columns"
+          :columns="columnLabels"
           :rows="filteredChannels"
           :loading="loading"
           pricing-key-prefix="availableChannels.pricing"
@@ -65,22 +65,37 @@ const channels = ref<UserAvailableChannel[]>([])
 const loading = ref(false)
 const searchQuery = ref('')
 
-const columns = computed(() => [
-  { key: 'name', label: t('availableChannels.columns.name') },
-  { key: 'groups', label: t('availableChannels.columns.groups') },
-  { key: 'supported_models', label: t('availableChannels.columns.supportedModels') }
-])
+const columnLabels = computed(() => ({
+  name: t('availableChannels.columns.name'),
+  platform: t('availableChannels.columns.platform'),
+  groups: t('availableChannels.columns.groups'),
+  supportedModels: t('availableChannels.columns.supportedModels'),
+}))
 
+/**
+ * 搜索过滤：
+ * - 命中渠道名/描述 → 整个渠道（所有 platforms）都保留
+ * - 否则按 platform/group/model 维度在 sections 里过滤，保留有匹配的 section
+ * - 所有 sections 都不匹配时，渠道本身被过滤掉
+ */
 const filteredChannels = computed(() => {
   const q = searchQuery.value.trim().toLowerCase()
   if (!q) return channels.value
-  return channels.value.filter((ch) => {
-    if (ch.name.toLowerCase().includes(q)) return true
-    if ((ch.description || '').toLowerCase().includes(q)) return true
-    if (ch.groups.some((g) => g.name.toLowerCase().includes(q))) return true
-    if (ch.supported_models.some((m) => m.name.toLowerCase().includes(q))) return true
-    return false
-  })
+  return channels.value
+    .map((ch) => {
+      const nameHit = ch.name.toLowerCase().includes(q)
+      const descHit = (ch.description || '').toLowerCase().includes(q)
+      if (nameHit || descHit) return ch
+      const matchingSections = ch.platforms.filter(
+        (p) =>
+          p.platform.toLowerCase().includes(q) ||
+          p.groups.some((g) => g.name.toLowerCase().includes(q)) ||
+          p.supported_models.some((m) => m.name.toLowerCase().includes(q)),
+      )
+      if (matchingSections.length === 0) return null
+      return { ...ch, platforms: matchingSections }
+    })
+    .filter((ch): ch is UserAvailableChannel => ch !== null)
 })
 
 async function loadChannels() {