Improve selected-text highlighting in the decompiler view

Ports icsharpcode/ILSpy#2938. Selected text kept the run's syntax colours by leaving the AvaloniaEdit TextArea SelectionForeground unset, and the selection is drawn as a flat, translucent highlight (square corners) rather than an opaque recolour. The brush is theme-aware -- #007ACC at 30% on the light editor background, a brighter #3794FF at 35% so it still reads on the dark one.

ILSpy/App.axaml

@@ -38,6 +38,9 @@
                      surfaces a reader spends most time looking at. -->
                 <ResourceDictionary x:Key="Light">
                     <SolidColorBrush x:Key="ILSpy.EditorBackground" Color="#FFFFE1" />
+                    <!-- Translucent selection so selected text keeps its syntax colours (the
+                         TextArea style leaves SelectionForeground unset). -->
+                    <SolidColorBrush x:Key="ILSpy.EditorSelectionBrush" Color="#007ACC" Opacity="0.3" />
                     <SolidColorBrush x:Key="ILSpy.EditorWaitAdornerBackground" Color="#80FFFFFF" />
                     <SolidColorBrush x:Key="ILSpy.ToolbarBackground" Color="White" />
                     <SolidColorBrush x:Key="ILSpy.ToolbarBorder" Color="#FFA9B0B7" />
@@ -86,6 +89,9 @@
                 </ResourceDictionary>
                 <ResourceDictionary x:Key="Dark">
                     <SolidColorBrush x:Key="ILSpy.EditorBackground" Color="#1E1E1E" />
+                    <!-- Brighter, slightly stronger so the translucent selection still reads on the
+                         dark editor background. -->
+                    <SolidColorBrush x:Key="ILSpy.EditorSelectionBrush" Color="#3794FF" Opacity="0.35" />
                     <SolidColorBrush x:Key="ILSpy.EditorWaitAdornerBackground" Color="#A0000000" />
                     <SolidColorBrush x:Key="ILSpy.ToolbarBackground" Color="#2D2D30" />
                     <SolidColorBrush x:Key="ILSpy.ToolbarBorder" Color="#3F3F46" />

ILSpy/TextView/DecompilerTextView.axaml

@@ -3,12 +3,23 @@
              xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
              xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
              xmlns:ae="using:AvaloniaEdit"
+             xmlns:aeediting="using:AvaloniaEdit.Editing"
              xmlns:textView="using:ILSpy.TextView"
              mc:Ignorable="d" d:DesignWidth="600" d:DesignHeight="400"
              Name="self"
              x:Class="ILSpy.TextView.DecompilerTextView"
              x:DataType="textView:DecompilerTabPageModel">
 
+	<!-- Selected-text highlighting (ports icsharpcode/ILSpy#2938): keep the syntax colours of
+	     selected text by leaving SelectionForeground unset, and draw a flat, translucent
+	     highlight (square corners, no border) instead of recolouring the run. -->
+	<UserControl.Styles>
+		<Style Selector="aeediting|TextArea">
+			<Setter Property="SelectionCornerRadius" Value="0" />
+			<Setter Property="SelectionBrush" Value="{DynamicResource ILSpy.EditorSelectionBrush}" />
+		</Style>
+	</UserControl.Styles>
+
 	<!-- Wait-adorner: a translucent overlay above the editor with a centred title +
 	     progress bar + cancel button while a decompilation is in flight. -->
 	<Grid>