App 2.4.2: pairing dialog shows errors instead of closing

- On a failure the pairing dialog stays open and shows the reason in
  red (Cancel becomes Close); before, it closed at once and the reason
  was only in the Server tab's status line.
- "Pairing is not enabled" only when the server says so. Any other 404
  (a server older than 2.1.0, or a proxy's page) is reported as a
  server without pairing.
- Pairing and Test connection name the problem: a server name that
  does not resolve (Test connection used to say "No internet
  connection"), a server that does not answer, or the HTTP status.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QLJfZQZeAKAwSqViTmLzki

app/build.gradle

@@ -34,8 +34,8 @@ android {
         applicationId "com.stinnesbeck.nilsmg4"
         minSdk 27
         targetSdk 36
-        versionCode 59
-        versionName "2.4.1"
+        versionCode 60
+        versionName "2.4.2"
 
         // Nothing about the server ships in the APK: its URL and ingest token are entered in
         // the app's Server tab (ServerConfig), and the database is only reachable by the server.