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/MainWindow.axaml

@@ -6,9 +6,7 @@
         Height="500" Width="800"
         MinHeight="400" MinWidth="600"
         WindowStartupLocation="CenterScreen">
-    <Window.DataContext>
-        <vm:MainWindowViewModel />
-    </Window.DataContext>
+    <!-- DataContext is set via DI in App.axaml.cs -->
 
     <Window.KeyBindings>
         <KeyBinding Gesture="Ctrl+N" Command="{Binding NewStoreCommand}" />