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/src/Utils/JdeScoping.SecureStoreManager/ViewModels/SecretItemViewModel.cs

@@ -1,4 +1,5 @@
 using System.Windows.Input;
+using JdeScoping.SecureStoreManager.Services;
 
 namespace JdeScoping.SecureStoreManager.ViewModels;
 
@@ -8,13 +9,15 @@ namespace JdeScoping.SecureStoreManager.ViewModels;
 public class SecretItemViewModel : ViewModelBase
 {
     private readonly string _actualValue;
+    private readonly IClipboardService _clipboardService;
     private bool _isValueVisible;
     private const string MaskedValue = "********";
 
-    public SecretItemViewModel(string key, string value)
+    public SecretItemViewModel(string key, string value, IClipboardService clipboardService)
     {
         Key = key;
         _actualValue = value;
+        _clipboardService = clipboardService ?? throw new ArgumentNullException(nameof(clipboardService));
         ToggleVisibilityCommand = new RelayCommand(ToggleVisibility);
         CopyToClipboardCommand = new RelayCommand(CopyToClipboard);
     }
@@ -60,10 +63,9 @@ public class SecretItemViewModel : ViewModelBase
     public ICommand CopyToClipboardCommand { get; }
 
     /// <summary>
-    /// Event raised when clipboard copy is requested.
-    /// The view subscribes to this to perform the actual clipboard operation.
+    /// Event raised when clipboard copy fails.
     /// </summary>
-    public event Func<string, Task>? OnCopyToClipboard;
+    public event Action<string>? OnCopyFailed;
 
     private void ToggleVisibility()
     {
@@ -74,14 +76,11 @@ public class SecretItemViewModel : ViewModelBase
     {
         try
         {
-            if (OnCopyToClipboard != null)
-            {
-                await OnCopyToClipboard(_actualValue);
-            }
+            await _clipboardService.SetTextAsync(_actualValue);
         }
-        catch
+        catch (Exception ex)
         {
-            // Clipboard operations can fail in some scenarios
+            OnCopyFailed?.Invoke($"Failed to copy to clipboard: {ex.Message}");
         }
     }
 }