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

@@ -7,9 +7,7 @@
         WindowStartupLocation="CenterOwner"
         CanResize="False"
         ShowInTaskbar="False">
-    <Window.DataContext>
-        <vm:OpenStoreDialogViewModel />
-    </Window.DataContext>
+    <!-- DataContext is set in code-behind -->
 
     <Grid Margin="15" RowDefinitions="Auto,Auto,Auto,*,Auto">
         <!-- Store Path -->
@@ -87,7 +85,7 @@
 
         <!-- Buttons -->
         <StackPanel Grid.Row="4" Orientation="Horizontal" HorizontalAlignment="Right" Spacing="10">
-            <Button Content="Open" Click="OpenButton_Click" MinWidth="80" Padding="10,5" />
+            <Button Content="Open" Click="OpenButton_Click" IsEnabled="{Binding IsValid}" MinWidth="80" Padding="10,5" />
             <Button Content="Cancel" Click="CancelButton_Click" MinWidth="80" Padding="10,5" />
         </StackPanel>
     </Grid>