feat(channels): explode available channels by platform + apply platform theme

Backend: one source channel → N output rows, one per platform that
has user-visible groups. Each row carries a single platform, so the
frontend can color/icon an entire row without mixing sources.

- userAvailableChannel: add Platform field
- new explodeChannelByPlatform helper; drop now-redundant
  collectGroupPlatforms

Frontend: use the row platform to drive theming and stop repeating
"ANTHROPIC" / "OPENAI" labels on every model chip.

- api/channels.ts: UserAvailableChannel.platform
- AvailableChannelsTable: name cell — PlatformBadge next to channel
  name (replaces the two-line name/description block; description
  moves to the badge's title tooltip); groups cell — each chip uses
  platformBadgeLightClass + PlatformIcon; model list passes
  show-platform=false + platform-hint to child chips
- SupportedModelChip: chip bg/border driven by platformBadgeClass,
  leading PlatformIcon; platform-hint fallback when model.platform
  missing

backend/internal/handler/available_channel_handler_test.go

@@ -42,21 +42,6 @@ func TestFilterUserVisibleGroups_IntersectionOnly(t *testing.T) {
 	require.ElementsMatch(t, []int64{1, 3}, ids)
 }
 
-func TestCollectGroupPlatforms_DerivesAllowedSet(t *testing.T) {
-	groups := []userAvailableGroup{
-		{ID: 1, Platform: "anthropic"},
-		{ID: 2, Platform: "openai"},
-		{ID: 3, Platform: "anthropic"}, // 去重
-		{ID: 4, Platform: ""},          // 空平台忽略
-	}
-	got := collectGroupPlatforms(groups)
-	require.Len(t, got, 2)
-	_, hasAnt := got["anthropic"]
-	_, hasOA := got["openai"]
-	require.True(t, hasAnt)
-	require.True(t, hasOA)
-}
-
 func TestToUserSupportedModels_FiltersByAllowedPlatforms(t *testing.T) {
 	// 用户可访问分组只覆盖 anthropic；anthropic 平台的模型保留，openai 模型被剔除。
 	src := []service.SupportedModel{