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.PreviewPresentation.cs

@@ -514,61 +514,27 @@ public partial class ChatWindow
         if (_previewTabPopup != null)
             _previewTabPopup.IsOpen = false;
 
-        var bg = TryFindResource("LauncherBackground") as Brush ?? new SolidColorBrush(Color.FromRgb(0x1E, 0x1E, 0x2E));
-        var borderBrush = TryFindResource("BorderColor") as Brush ?? Brushes.Gray;
         var primaryText = TryFindResource("PrimaryText") as Brush ?? Brushes.White;
         var secondaryText = TryFindResource("SecondaryText") as Brush ?? Brushes.Gray;
-        var hoverBg = TryFindResource("ItemHoverBackground") as Brush ?? new SolidColorBrush(Color.FromArgb(0x18, 0xFF, 0xFF, 0xFF));
 
         var stack = new StackPanel();
 
         void AddItem(string icon, string iconColor, string label, Action action)
         {
-            var itemBorder = new Border
-            {
-                Background = Brushes.Transparent,
-                CornerRadius = new CornerRadius(6),
-                Padding = new Thickness(10, 7, 16, 7),
-                Cursor = Cursors.Hand,
-            };
-            var sp = new StackPanel { Orientation = Orientation.Horizontal };
-            sp.Children.Add(new TextBlock
-            {
-                Text = icon,
-                FontFamily = new FontFamily("Segoe MDL2 Assets"),
-                FontSize = 12,
-                Foreground = string.IsNullOrEmpty(iconColor)
-                    ? secondaryText
-                    : new SolidColorBrush((Color)ColorConverter.ConvertFromString(iconColor)),
-                VerticalAlignment = VerticalAlignment.Center,
-                Margin = new Thickness(0, 0, 8, 0),
-            });
-            sp.Children.Add(new TextBlock
-            {
-                Text = label,
-                FontSize = 13,
-                Foreground = primaryText,
-                VerticalAlignment = VerticalAlignment.Center,
-            });
-            itemBorder.Child = sp;
-            itemBorder.MouseEnter += (s, _) => { if (s is Border b) b.Background = hoverBg; };
-            itemBorder.MouseLeave += (s, _) => { if (s is Border b) b.Background = Brushes.Transparent; };
-            itemBorder.MouseLeftButtonUp += (_, _) =>
+            var iconBrush = string.IsNullOrEmpty(iconColor)
+                ? secondaryText
+                : new SolidColorBrush((Color)ColorConverter.ConvertFromString(iconColor));
+            var itemBorder = CreateSurfacePopupMenuItem(icon, iconBrush, label, () =>
             {
                 _previewTabPopup!.IsOpen = false;
                 action();
-            };
+            }, primaryText, 13);
             stack.Children.Add(itemBorder);
         }
 
         void AddSeparator()
         {
-            stack.Children.Add(new Border
-            {
-                Height = 1,
-                Background = borderBrush,
-                Margin = new Thickness(8, 3, 8, 3),
-            });
+            stack.Children.Add(CreateSurfacePopupSeparator());
         }
 
         AddItem("\uE8A7", "#64B5F6", "외부 프로그램으로 열기", () =>
@@ -628,23 +594,7 @@ public partial class ChatWindow
             });
         }
 
-        var popupBorder = new Border
-        {
-            Background = bg,
-            BorderBrush = borderBrush,
-            BorderThickness = new Thickness(1),
-            CornerRadius = new CornerRadius(12),
-            Padding = new Thickness(4, 6, 4, 6),
-            MinWidth = 180,
-            Effect = new System.Windows.Media.Effects.DropShadowEffect
-            {
-                BlurRadius = 16,
-                Opacity = 0.4,
-                ShadowDepth = 4,
-                Color = Colors.Black,
-            },
-            Child = stack,
-        };
+        var popupBorder = CreateSurfacePopupContainer(stack, 180, new Thickness(4, 6, 4, 6));
 
         _previewTabPopup = new Popup
         {