fix(client-ui): resolve Medium code-review finding (Client.UI-007)

Remove Password from UserSettings and stop writing it to settings.json;
the operator is re-prompted on each launch. Update LoadSettings/SaveSettings
comments and adjust the affected test assertion to verify the password is
not restored from the persisted model.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/Client/ZB.MOM.WW.OtOpcUa.Client.UI.Tests/MainWindowViewModelTests.cs

@@ -438,7 +438,6 @@ public class MainWindowViewModelTests
         {
             EndpointUrl = "opc.tcp://saved:5555",
             Username = "savedUser",
-            Password = "savedPass",
             SecurityMode = SecurityMode.Sign,
             FailoverUrls = "opc.tcp://backup:5555",
             SessionTimeoutSeconds = 120,
@@ -458,7 +457,8 @@ public class MainWindowViewModelTests
 
         vm.EndpointUrl.ShouldBe("opc.tcp://saved:5555");
         vm.Username.ShouldBe("savedUser");
-        vm.Password.ShouldBe("savedPass");
+        // Password is intentionally not persisted: re-prompt each launch
+        vm.Password.ShouldBeNull();
         vm.SelectedSecurityMode.ShouldBe(SecurityMode.Sign);
         vm.FailoverUrls.ShouldBe("opc.tcp://backup:5555");
         vm.SessionTimeoutSeconds.ShouldBe(120);