fix: Update StackPanel Spacing styles in WpfUiCustom.xaml theme

## Problem
The WpfUiCustom.xaml theme file contained 3 StackPanel styles that used
Property="Spacing", which doesn't exist in WPF StackPanel:
- StandardSpacing (Spacing=12)
- CompactSpacing (Spacing=8)
- WideSpacing (Spacing=16)

These styles were causing XAML build errors at lines 68, 72, and 76.

## Solution
1. Added xmlns:helpers namespace to ResourceDictionary with assembly reference
2. Updated all 3 style definitions to use helpers:StackPanelHelper.Spacing
   attached property instead of Property="Spacing"

## Files Changed
- Themes/WpfUiCustom.xaml

## Result
All StackPanel spacing styles now use the WPF-compatible StackPanelHelper
attached property, maintaining the same spacing values and functionality.

Author: claude

Themes/WpfUiCustom.xaml

@@ -1,6 +1,7 @@
 <ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                     xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
-                    xmlns:ui="http://schemas.lepo.co/wpfui/2022/xaml">
+                    xmlns:ui="http://schemas.lepo.co/wpfui/2022/xaml"
+                    xmlns:helpers="clr-namespace:PrettyScreenSHOT.Helpers;assembly=PrettyScreenSHOT">
 
     <!--
     Custom WPF UI Styles for PrettyScreenSHOT
@@ -63,17 +64,17 @@
         <Setter Property="Margin" Value="0,0,0,8"/>
     </Style>
 
-    <!-- StackPanel with consistent spacing -->
+    <!-- StackPanel with consistent spacing using Attached Property -->
     <Style x:Key="StandardSpacing" TargetType="StackPanel">
-        <Setter Property="Spacing" Value="12"/>
+        <Setter Property="helpers:StackPanelHelper.Spacing" Value="12"/>
     </Style>
 
     <Style x:Key="CompactSpacing" TargetType="StackPanel">
-        <Setter Property="Spacing" Value="8"/>
+        <Setter Property="helpers:StackPanelHelper.Spacing" Value="8"/>
     </Style>
 
     <Style x:Key="WideSpacing" TargetType="StackPanel">
-        <Setter Property="Spacing" Value="16"/>
+        <Setter Property="helpers:StackPanelHelper.Spacing" Value="16"/>
     </Style>
 
     <!-- Section separator -->