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/ja-JP.json

@@ -1852,7 +1852,10 @@
         "noServers": "選択した国のサーバーが見つかりません",
         "noPublicKey": "選択したサーバーは NordLynx 公開鍵を公開していません。",
         "outboundAdded": "NordVPN アウトバウンドを追加しました",
-        "outboundUpdated": "NordVPN アウトバウンドを更新しました"
+        "outboundUpdated": "NordVPN アウトバウンドを更新しました",
+        "serverLoad": "サーバー負荷",
+        "addedServers": "追加済みサーバー",
+        "alreadyAdded": "このサーバーはすでにアウトバウンド一覧にあります。{reset} で鍵を更新してください。"
       },
       "warp": {
         "changeIp": "IP を変更",