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/en-US.json

@@ -1970,7 +1970,10 @@
         "noServers": "No servers found for the selected country",
         "noPublicKey": "Selected server does not advertise a NordLynx public key.",
         "outboundAdded": "NordVPN outbound added",
-        "outboundUpdated": "NordVPN outbound updated"
+        "outboundUpdated": "NordVPN outbound updated",
+        "serverLoad": "Server load",
+        "addedServers": "Added servers",
+        "alreadyAdded": "This server is already in the outbound list. Use {reset} to refresh its key."
       },
       "warp": {
         "changeIp": "Change IP",