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/ja-JP.json

@@ -1866,7 +1866,9 @@
         "edit": "フェイクDNS編集",
         "ipPool": "IPプールサブネット",
         "poolSize": "プールサイズ"
-      }
+      },
+      "defaultOutbound": "デフォルトアウトバウンド",
+      "defaultOutboundDesc": "ルーティング規則に一致しないトラフィックはこのアウトバウンドを使います（一覧の先頭）。"
     },
     "hosts": {
       "addHost": "ホストを追加",