🔔 feat: Add subscription-aware quota notifications and update UI copy

Routes quota alerts through a subscription-specific check when billing from subscriptions, preventing wallet-based thresholds from triggering false warnings.
Updates the notification settings description and localization keys to clarify that both wallet and subscription balances are monitored.

service/billing.go

@@ -58,9 +58,13 @@ func SettleBilling(ctx *gin.Context, relayInfo *relaycommon.RelayInfo, actualQuo
 			return err
 		}
 
-		// 发送额度通知
+		// 发送额度通知（订阅计费使用订阅剩余额度）
 		if actualQuota != 0 {
-			checkAndSendQuotaNotify(relayInfo, actualQuota-preConsumed, preConsumed)
+			if relayInfo.BillingSource == BillingSourceSubscription {
+				checkAndSendSubscriptionQuotaNotify(relayInfo)
+			} else {
+				checkAndSendQuotaNotify(relayInfo, actualQuota-preConsumed, preConsumed)
+			}
 		}
 		return nil
 	}