feat: move test buttons to right panel, add user auth dialog for reports

Move Start/Stop/Report buttons from the middle panel to the top of
TestPanelView (automated tests section), matching the old application
layout. Remove inline Operator/Client text fields — operator identity
now comes from a UserCheckDialog (username/password) shown before the
existing ReportDialog. Add credential storage to ConfigurationService.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Services/IConfigurationService.cs

@@ -62,5 +62,16 @@ namespace HC_APTBS.Services
 
         /// <summary>Saves updated sensor calibration data to <c>sensors.xml</c>.</summary>
         void SaveSensors();
+
+        // ── Users ─────────────────────────────────────────────────────────────────
+
+        /// <summary>Validates a username/password pair against stored credentials.</summary>
+        bool ValidateUser(string username, string password);
+
+        /// <summary>Returns all stored user credentials as a dictionary.</summary>
+        IReadOnlyDictionary<string, string> GetUsers();
+
+        /// <summary>Replaces all stored user credentials and persists them.</summary>
+        void UpdateUsers(Dictionary<string, string> users);
     }
 }