AX Agent surface visual language 공통화 및 preview/file browser 정리

- ChatWindow에 공통 popup container/menu item/separator/file tree header helper를 추가
- Preview와 FileBrowser presentation이 같은 surface 스타일을 사용하도록 정리
- README와 DEVELOPMENT 문서에 2026-04-06 11:20 (KST) 기준 visual polish 1차 반영
- dotnet build 검증 경고 0, 오류 0 확인

src/AxCopilot/Views/ChatWindow.SurfaceVisualPresentation.cs

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+using System;
+using System.Windows;
+using System.Windows.Controls;
+using System.Windows.Controls.Primitives;
+using System.Windows.Input;
+using System.Windows.Media;
+
+namespace AxCopilot.Views;
+
+public partial class ChatWindow
+{
+    private Border CreateSurfacePopupContainer(UIElement content, double minWidth = 180, Thickness? padding = null)
+    {
+        var bg = TryFindResource("LauncherBackground") as Brush ?? new SolidColorBrush(Color.FromRgb(0x1E, 0x1E, 0x2E));
+        var borderBrush = TryFindResource("BorderColor") as Brush ?? Brushes.Gray;
+
+        return new Border
+        {
+            Background = bg,
+            BorderBrush = borderBrush,
+            BorderThickness = new Thickness(1),
+            CornerRadius = new CornerRadius(12),
+            Padding = padding ?? new Thickness(6),
+            MinWidth = minWidth,
+            Effect = new System.Windows.Media.Effects.DropShadowEffect
+            {
+                BlurRadius = 16,
+                ShadowDepth = 4,
+                Opacity = 0.34,
+                Color = Colors.Black,
+            },
+            Child = content,
+        };
+    }
+
+    private Border CreateSurfacePopupMenuItem(string icon, Brush iconBrush, string label, Action onClick, Brush? labelBrush = null, double fontSize = 12.5)
+    {
+        var primaryText = TryFindResource("PrimaryText") as Brush ?? Brushes.White;
+        var hoverBg = TryFindResource("ItemHoverBackground") as Brush ?? new SolidColorBrush(Color.FromArgb(0x18, 0xFF, 0xFF, 0xFF));
+
+        var item = new Border
+        {
+            Background = Brushes.Transparent,
+            CornerRadius = new CornerRadius(8),
+            Padding = new Thickness(10, 8, 14, 8),
+            Margin = new Thickness(0, 1, 0, 1),
+            Cursor = Cursors.Hand,
+        };
+
+        var row = new StackPanel { Orientation = Orientation.Horizontal };
+        row.Children.Add(new TextBlock
+        {
+            Text = icon,
+            FontFamily = new FontFamily("Segoe MDL2 Assets"),
+            FontSize = 13,
+            Foreground = iconBrush,
+            VerticalAlignment = VerticalAlignment.Center,
+            Margin = new Thickness(0, 0, 10, 0),
+        });
+        row.Children.Add(new TextBlock
+        {
+            Text = label,
+            FontSize = fontSize,
+            Foreground = labelBrush ?? primaryText,
+            VerticalAlignment = VerticalAlignment.Center,
+        });
+        item.Child = row;
+
+        item.MouseEnter += (s, _) => { if (s is Border b) b.Background = hoverBg; };
+        item.MouseLeave += (s, _) => { if (s is Border b) b.Background = Brushes.Transparent; };
+        item.MouseLeftButtonUp += (_, _) => onClick();
+        return item;
+    }
+
+    private Border CreateSurfacePopupSeparator()
+    {
+        var borderBrush = TryFindResource("BorderColor") as Brush ?? Brushes.Gray;
+        return new Border
+        {
+            Height = 1,
+            Margin = new Thickness(10, 4, 10, 4),
+            Background = borderBrush,
+            Opacity = 0.3,
+        };
+    }
+
+    private StackPanel CreateSurfaceFileTreeHeader(string icon, string name, string? sizeText)
+    {
+        var secondaryText = TryFindResource("SecondaryText") as Brush ?? Brushes.Gray;
+
+        var sp = new StackPanel { Orientation = Orientation.Horizontal };
+        sp.Children.Add(new TextBlock
+        {
+            Text = icon,
+            FontFamily = new FontFamily("Segoe MDL2 Assets"),
+            FontSize = 11,
+            Foreground = secondaryText,
+            VerticalAlignment = VerticalAlignment.Center,
+            Margin = new Thickness(0, 0, 5, 0),
+        });
+        sp.Children.Add(new TextBlock
+        {
+            Text = name,
+            FontSize = 11.5,
+            Foreground = TryFindResource("PrimaryText") as Brush ?? Brushes.White,
+            VerticalAlignment = VerticalAlignment.Center,
+        });
+        if (!string.IsNullOrEmpty(sizeText))
+        {
+            sp.Children.Add(new TextBlock
+            {
+                Text = $"  {sizeText}",
+                FontSize = 10,
+                Foreground = secondaryText,
+                VerticalAlignment = VerticalAlignment.Center,
+            });
+        }
+
+        return sp;
+    }
+}