fix: restore speedtest.uztelecom.uz as the default server

Consolidating the UI's seed values onto TestConfiguration's own defaults also
silently changed the default target to iperf.es.net, which was not asked for.
Sets it back in TestConfiguration, which is now the single definition the
screen and the headless entry point both read.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

lib/domain/models/test_configuration.dart

@@ -7,7 +7,7 @@ class TestConfiguration {
   final int parallelStreams;
 
   const TestConfiguration({
-    this.server = 'iperf.es.net',
+    this.server = 'speedtest.uztelecom.uz',
     this.port = 5201,
     this.duration = const Duration(seconds: 10),
     this.interval = 1,