Enhance FlexPanel layout system with Grow, Spacing, and alignment improvements (#15)

- Improved JustifyContent logic, removed deprecated SpaceAuto
- Added AlignContent support for SpaceBetween, SpaceAround, and SpaceEvenly
- Implemented AlignItems (Start, Center, End, Stretch)
- Introduced Spacing property (disabled for Space* alignments)
- Added Grow (flex-grow) support for proportional resizing
- Changed default layout behavior to Wrap mode
- Preview layout improved for better alignment visualization
- Added caution for layout inconsistencies when mixing unsupported alignment combinations

Author: lukewire129

src/OpenSilver.ControlsKit.Controls/FlexPanel.Grow.cs

@@ -0,0 +1,19 @@
+using System;
+using System.Windows;
+using System.Windows.Controls;
+
+namespace OpenSilver.ControlsKit
+{
+    public partial class FlexPanel : Panel
+    {
+        public static readonly DependencyProperty GrowProperty =
+                    DependencyProperty.RegisterAttached (
+        "Grow",
+        typeof (double),
+        typeof (FlexPanel),
+        new FrameworkPropertyMetadata (0.0, FrameworkPropertyMetadataOptions.AffectsMeasure | FrameworkPropertyMetadataOptions.AffectsArrange));
+
+        public static void SetGrow(UIElement element, double value) => element.SetValue (GrowProperty, value);
+        public static double GetGrow(UIElement element) => (double)element.GetValue (GrowProperty);
+    }
+}