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/ViewModels/MainWindowViewModelTests.cs

@@ -9,12 +9,16 @@ namespace JdeScoping.SecureStoreManager.Tests.ViewModels;
 public class MainWindowViewModelTests
 {
     private readonly ISecureStoreManager _mockStoreManager;
+    private readonly IDialogService _mockDialogService;
+    private readonly IClipboardService _mockClipboardService;
     private readonly MainWindowViewModel _sut;
 
     public MainWindowViewModelTests()
     {
         _mockStoreManager = Substitute.For<ISecureStoreManager>();
-        _sut = new MainWindowViewModel(_mockStoreManager);
+        _mockDialogService = Substitute.For<IDialogService>();
+        _mockClipboardService = Substitute.For<IClipboardService>();
+        _sut = new MainWindowViewModel(_mockStoreManager, _mockDialogService, _mockClipboardService);
     }
 
     [Fact]
@@ -280,7 +284,7 @@ public class MainWindowViewModelTests
         };
 
         // Act
-        _sut.SelectedSecret = new SecretItemViewModel("key", "value");
+        _sut.SelectedSecret = new SecretItemViewModel("key", "value", _mockClipboardService);
 
         // Assert
         propertyChangedRaised.ShouldBeTrue();