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>

ViewModels/Dialogs/UserCheckViewModel.cs

@@ -0,0 +1,81 @@
+using System.Windows;
+using CommunityToolkit.Mvvm.ComponentModel;
+using CommunityToolkit.Mvvm.Input;
+using HC_APTBS.Services;
+
+namespace HC_APTBS.ViewModels.Dialogs
+{
+    /// <summary>
+    /// ViewModel for the user authentication dialog shown before report generation.
+    /// Validates operator credentials against the stored user list.
+    /// </summary>
+    public sealed partial class UserCheckViewModel : ObservableObject
+    {
+        private readonly IConfigurationService _config;
+
+        /// <summary>Initialises the dialog, optionally pre-filling the last used username.</summary>
+        public UserCheckViewModel(IConfigurationService config, string lastUsername = "")
+        {
+            _config   = config;
+            _username = lastUsername;
+        }
+
+        // ── Bindable properties ───────────────────────────────────────────────────
+
+        /// <summary>Username entered by the operator.</summary>
+        [ObservableProperty]
+        [NotifyCanExecuteChangedFor(nameof(AcceptCommand))]
+        private string _username = string.Empty;
+
+        /// <summary>Password entered by the operator (set from code-behind PasswordChanged handler).</summary>
+        [ObservableProperty]
+        [NotifyCanExecuteChangedFor(nameof(AcceptCommand))]
+        private string _password = string.Empty;
+
+        // ── Dialog result ─────────────────────────────────────────────────────────
+
+        /// <summary>True if the user authenticated successfully.</summary>
+        public bool Accepted { get; private set; }
+
+        /// <summary>The validated username, available after <see cref="Accepted"/> is true.</summary>
+        public string AuthenticatedUser { get; private set; } = string.Empty;
+
+        // ── Commands ──────────────────────────────────────────────────────────────
+
+        /// <summary>Validates credentials and closes the dialog on success.</summary>
+        [RelayCommand(CanExecute = nameof(CanAccept))]
+        private void Accept()
+        {
+            if (_config.ValidateUser(Username, Password))
+            {
+                AuthenticatedUser = Username;
+                Accepted = true;
+                RequestClose?.Invoke();
+            }
+            else
+            {
+                MessageBox.Show(
+                    "Invalid username or password.\n(Both are case-sensitive.)",
+                    "Authentication Error",
+                    MessageBoxButton.OK,
+                    MessageBoxImage.Stop);
+            }
+        }
+
+        private bool CanAccept() => !string.IsNullOrWhiteSpace(Username)
+                                 && !string.IsNullOrEmpty(Password);
+
+        /// <summary>Cancels authentication and closes the dialog.</summary>
+        [RelayCommand]
+        private void Cancel()
+        {
+            Accepted = false;
+            RequestClose?.Invoke();
+        }
+
+        // ── Events ────────────────────────────────────────────────────────────────
+
+        /// <summary>Raised when the dialog should close itself.</summary>
+        public event System.Action? RequestClose;
+    }
+}