Style: global 4px margin on all interactive/display controls in Avalonia app

Single style rule in App.axaml targeting Button, TextBox, ComboBox, CheckBox,
ListBox, SelectableTextBlock, and TabControl so every control gets consistent
4px spacing without touching individual XAML files.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: sharpninja

src/RemoteAgent.Desktop/App.axaml

@@ -41,5 +41,9 @@
     <fluent:FluentTheme />
     <fa:FluentAvaloniaTheme />
     <StyleInclude Source="avares://ToolBarControls.Avalonia/Themes/ToolBarControls.Avalonia.axaml" />
+    <!-- Global 4 px margin on all interactive / display controls -->
+    <Style Selector="Button, TextBox, ComboBox, CheckBox, ListBox, SelectableTextBlock, TabControl">
+      <Setter Property="Margin" Value="4" />
+    </Style>
   </Application.Styles>
 </Application>