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/Views/OpenStoreDialog.axaml.cs

@@ -1,6 +1,7 @@
 using Avalonia.Controls;
 using Avalonia.Interactivity;
 using Avalonia.Platform.Storage;
+using JdeScoping.SecureStoreManager.Constants;
 using JdeScoping.SecureStoreManager.ViewModels;
 using MsBox.Avalonia;
 using MsBox.Avalonia.Enums;
@@ -14,6 +15,7 @@ public partial class OpenStoreDialog : Window
     public OpenStoreDialog()
     {
         InitializeComponent();
+        DataContext = new OpenStoreDialogViewModel();
         Loaded += OpenStoreDialog_Loaded;
     }
 
@@ -31,7 +33,7 @@ public partial class OpenStoreDialog : Window
             FileTypeFilter = new[]
             {
                 new FilePickerFileType(fileTypeName) { Patterns = new[] { pattern } },
-                new FilePickerFileType("All Files") { Patterns = new[] { "*.*" } }
+                new FilePickerFileType(FileExtensions.AllFilesTypeName) { Patterns = new[] { FileExtensions.AllFilesPattern } }
             }
         });
 
@@ -44,8 +46,8 @@ public partial class OpenStoreDialog : Window
         {
             var box = MessageBoxManager
                 .GetMessageBoxStandard(
-                    "Validation Error",
-                    ViewModel.ValidationError ?? "Please fill in all required fields.",
+                    DialogStrings.ValidationErrorTitle,
+                    ViewModel.ValidationError ?? DialogStrings.DefaultValidationError,
                     ButtonEnum.Ok,
                     MsBox.Avalonia.Enums.Icon.Warning);
             await box.ShowWindowDialogAsync(this);