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>

src/Client/ZB.MOM.WW.OtOpcUa.Client.UI/Services/UserSettings.cs

@@ -17,11 +17,6 @@ public sealed class UserSettings
     /// </summary>
     public string? Username { get; set; }
 
-    /// <summary>
-    /// Gets or sets the persisted password for authenticated sessions.
-    /// </summary>
-    public string? Password { get; set; }
-
     /// <summary>
     /// Gets or sets the transport security mode selected by the user.
     /// </summary>