fix: validate empty intervals + antigravity platform pricing match

- Backend: reject intervals with all-null price fields on save
- Backend: filterValidIntervals skips empty intervals in pricing resolver
- Frontend: red border + asterisk on empty interval rows
- Backend: antigravity groups now match anthropic/gemini channel pricing

backend/internal/handler/admin/channel_handler.go

@@ -2,6 +2,7 @@ package admin
 
 import (
 	"errors"
+	"fmt"
 	"strconv"
 	"strings"
 
@@ -233,10 +234,26 @@ func validatePricingBillingMode(pricing []service.ChannelModelPricing) error {
 				return errors.New("per-request price or intervals required for per_request/image billing mode")
 			}
 		}
+		// 校验 interval：至少有一个价格字段非空
+		for _, iv := range p.Intervals {
+			if iv.InputPrice == nil && iv.OutputPrice == nil &&
+				iv.CacheWritePrice == nil && iv.CacheReadPrice == nil &&
+				iv.PerRequestPrice == nil {
+				return fmt.Errorf("interval [%d, %s] has no price fields set for model %v",
+					iv.MinTokens, formatMaxTokens(iv.MaxTokens), p.Models)
+			}
+		}
 	}
 	return nil
 }
 
+func formatMaxTokens(max *int) string {
+	if max == nil {
+		return "∞"
+	}
+	return fmt.Sprintf("%d", *max)
+}
+
 // --- Handlers ---
 
 // List handles listing channels with pagination