refactor(securestoremanager): add platform service abstractions and constants

Implement deferred code review findings:
- Add IDialogService/IClipboardService interfaces for testable platform operations
- Create AvaloniaDialogService and AvaloniaClipboardService implementations
- Extract dialog strings and file extensions to centralized Constants classes
- Refactor ViewModels to use DI instead of event delegates
- Update tests to use mock services

NEW/tests/JdeScoping.SecureStoreManager.Tests/Views/MainWindowTests.cs

@@ -12,14 +12,27 @@ namespace JdeScoping.SecureStoreManager.Tests.Views;
 
 public class MainWindowTests
 {
+    private static MainWindowViewModel CreateViewModel(ISecureStoreManager? storeManager = null)
+    {
+        var mockStoreManager = storeManager ?? Substitute.For<ISecureStoreManager>();
+        var mockDialogService = Substitute.For<IDialogService>();
+        var mockClipboardService = Substitute.For<IClipboardService>();
+        return new MainWindowViewModel(mockStoreManager, mockDialogService, mockClipboardService);
+    }
+
     [AvaloniaFact]
     public void MainWindow_ShowsWithCorrectDefaultTitle()
     {
-        // Arrange & Act
-        var window = new MainWindow();
+        // Arrange
+        var storeManager = Substitute.For<ISecureStoreManager>();
+        storeManager.IsStoreOpen.Returns(false);
+        var viewModel = CreateViewModel(storeManager);
+        var window = new MainWindow { DataContext = viewModel };
+
+        // Act
         window.Show();
 
-        // Assert
+        // Assert - Title is bound to WindowTitle which is "SecureStore Manager" when no store is open
         window.Title.ShouldBe("SecureStore Manager");
     }
 
@@ -48,11 +61,14 @@ public class MainWindowTests
     [AvaloniaFact]
     public void MainWindow_DataContextIsMainWindowViewModel()
     {
-        // Arrange & Act
-        var window = new MainWindow();
+        // Arrange
+        var viewModel = CreateViewModel();
+
+        // Act
+        var window = new MainWindow { DataContext = viewModel };
         window.Show();
 
-        // Assert
+        // Assert - DataContext is set via DI in production, but must be set manually in tests
         window.DataContext.ShouldBeOfType<MainWindowViewModel>();
     }
 
@@ -103,31 +119,36 @@ public class MainWindowTests
     [AvaloniaFact]
     public void MainWindow_NewButtonCommand_IsBoundToViewModel()
     {
-        // Arrange & Act
-        var window = new MainWindow();
+        // Arrange
+        var storeManager = Substitute.For<ISecureStoreManager>();
+        var viewModel = CreateViewModel(storeManager);
+        var window = new MainWindow { DataContext = viewModel };
+
+        // Act
         window.Show();
 
-        var viewModel = window.DataContext as MainWindowViewModel;
         var buttons = window.GetVisualDescendants().OfType<Button>().ToList();
         var newButton = buttons.FirstOrDefault(b => b.Content?.ToString() == "New");
 
         // Assert
         newButton.ShouldNotBeNull();
-        newButton.Command.ShouldBe(viewModel?.NewStoreCommand);
+        newButton.Command.ShouldBe(viewModel.NewStoreCommand);
     }
 
     [AvaloniaFact]
     public void MainWindow_StatusBar_DisplaysStatusMessage()
     {
-        // Arrange & Act
-        var window = new MainWindow();
-        window.Show();
+        // Arrange
+        var storeManager = Substitute.For<ISecureStoreManager>();
+        var viewModel = CreateViewModel(storeManager);
+        var window = new MainWindow { DataContext = viewModel };
 
-        var viewModel = window.DataContext as MainWindowViewModel;
+        // Act
+        window.Show();
 
         // Assert - Find status bar text blocks
         var textBlocks = window.GetVisualDescendants().OfType<TextBlock>().ToList();
         // Status message should be "Ready" by default
-        textBlocks.Any(tb => tb.Text == viewModel?.StatusMessage).ShouldBeTrue();
+        textBlocks.Any(tb => tb.Text == viewModel.StatusMessage).ShouldBeTrue();
     }
 }