feat(xray): default outbound in basic routing (#5815)

* feat(xray): default outbound picker in basic routing

Let panel users choose which outbound handles unmatched traffic by
moving it to the first position in the template outbounds list.

* fix(xray): keep direct/blocked outbounds when changing default

* style(routing): revert incidental whitespace churn

Drop double blank lines and the reformatted function signature so the default-outbound diff stays focused on behavior.

internal/web/translation/zh-CN.json

@@ -1866,7 +1866,9 @@
         "edit": "编辑假 DNS",
         "ipPool": "IP 池子网",
         "poolSize": "池大小"
-      }
+      },
+      "defaultOutbound": "默认出站",
+      "defaultOutboundDesc": "未匹配任何路由规则的流量走此出站（列表中的第一个出站）。"
     },
     "hosts": {
       "addHost": "添加主机",