feat(nord): support multi-server NordLynx outbounds (#6311)

* feat(nord): support multi-server NordLynx outbounds

* fix(nord): address verified PR review findings

Tighten the NordVPN multi-outbound implementation and its regression
coverage based on the verified review feedback.

- remove the redundant Xray validation test that duplicated the base
  branch and did not exercise multiple outbounds
- make NordModal tests wait for server loading and assert the modal close
  callback, duplicate-server state, and endpoint behavior
- add coverage for resolving the NordLynx public key from technology
  metadata instead of a numeric technology ID
- use a real httptest server for Nord integration tests through an
  injectable API base URL
- represent the All Cities sentinel consistently as null and reset it
  when a country changes

The existing NordVPN API contracts and persisted outbound schema remain
unchanged.

internal/web/translation/zh-CN.json

@@ -1852,7 +1852,10 @@
         "noServers": "未找到选定国家/地区的服务器",
         "noPublicKey": "选定的服务器未公布 NordLynx 公钥。",
         "outboundAdded": "NordVPN 出站已添加",
-        "outboundUpdated": "NordVPN 出站已更新"
+        "outboundUpdated": "NordVPN 出站已更新",
+        "serverLoad": "服务器负载",
+        "addedServers": "已添加的服务器",
+        "alreadyAdded": "此服务器已在出站列表中。请使用{reset}刷新其密钥。"
       },
       "warp": {
         "changeIp": "更换 IP",