refactor(channels): tighten types and error paths per second review

- service: drop groupRepo nil guard (DI must inject), switch SupportedModels to SliceStable to match doc
- frontend: reuse user-side DTO types in SupportedModelChip/AvailableChannelsTable instead of duplicating shapes; narrow admin statusLabel param to ChannelStatus
- tests: replace nil-groupRepo case with ListAll/ListActive error propagation and BillingModelSource default-backfill coverage

backend/internal/service/channel.go

@@ -509,7 +509,7 @@ func (c *Channel) SupportedModels() []SupportedModel {
 		}
 	}
 
-	sort.Slice(result, func(i, j int) bool {
+	sort.SliceStable(result, func(i, j int) bool {
 		if result[i].Platform != result[j].Platform {
 			return result[i].Platform < result[j].Platform
 		}