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.FileBrowserPresentation.cs

@@ -94,7 +94,7 @@ public partial class ChatWindow
                 count++;
                 var dirItem = new TreeViewItem
                 {
-                    Header = CreateFileTreeHeader("\uED25", subDir.Name, null),
+                    Header = CreateSurfaceFileTreeHeader("\uED25", subDir.Name, null),
                     Tag = subDir.FullName,
                     IsExpanded = depth < 1,
                 };
@@ -140,7 +140,7 @@ public partial class ChatWindow
 
                 var fileItem = new TreeViewItem
                 {
-                    Header = CreateFileTreeHeader(icon, file.Name, size),
+                    Header = CreateSurfaceFileTreeHeader(icon, file.Name, size),
                     Tag = file.FullName,
                 };
 
@@ -167,38 +167,6 @@ public partial class ChatWindow
         }
     }
 
-    private static StackPanel CreateFileTreeHeader(string icon, string name, string? sizeText)
-    {
-        var sp = new StackPanel { Orientation = Orientation.Horizontal };
-        sp.Children.Add(new TextBlock
-        {
-            Text = icon,
-            FontFamily = new FontFamily("Segoe MDL2 Assets"),
-            FontSize = 11,
-            Foreground = new SolidColorBrush(Color.FromRgb(0x9C, 0xA3, 0xAF)),
-            VerticalAlignment = VerticalAlignment.Center,
-            Margin = new Thickness(0, 0, 5, 0),
-        });
-        sp.Children.Add(new TextBlock
-        {
-            Text = name,
-            FontSize = 11.5,
-            VerticalAlignment = VerticalAlignment.Center,
-        });
-        if (sizeText != null)
-        {
-            sp.Children.Add(new TextBlock
-            {
-                Text = $"  {sizeText}",
-                FontSize = 10,
-                Foreground = new SolidColorBrush(Color.FromRgb(0x6B, 0x72, 0x80)),
-                VerticalAlignment = VerticalAlignment.Center,
-            });
-        }
-
-        return sp;
-    }
-
     private static string GetFileIcon(string ext) => ext switch
     {
         ".html" or ".htm" => "\uEB41",
@@ -224,11 +192,8 @@ public partial class ChatWindow
 
     private void ShowFileTreeContextMenu(string filePath)
     {
-        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("HintBackground") as Brush ?? new SolidColorBrush(Color.FromArgb(0x18, 0xFF, 0xFF, 0xFF));
         var dangerBrush = new SolidColorBrush(Color.FromRgb(0xE7, 0x4C, 0x3C));
 
         var popup = new Popup
@@ -239,73 +204,22 @@ public partial class ChatWindow
             Placement = PlacementMode.MousePoint,
         };
         var panel = new StackPanel { Margin = new Thickness(2) };
-        var container = new Border
-        {
-            Background = bg,
-            BorderBrush = borderBrush,
-            BorderThickness = new Thickness(1),
-            CornerRadius = new CornerRadius(10),
-            Padding = new Thickness(6),
-            MinWidth = 200,
-            Effect = new System.Windows.Media.Effects.DropShadowEffect
-            {
-                BlurRadius = 16,
-                ShadowDepth = 4,
-                Opacity = 0.3,
-                Color = Colors.Black,
-                Direction = 270,
-            },
-            Child = panel,
-        };
+        var container = CreateSurfacePopupContainer(panel, 200, new Thickness(6));
         popup.Child = container;
 
         void AddItem(string icon, string label, Action action, Brush? labelColor = null, Brush? iconColor = null)
         {
-            var sp = new StackPanel { Orientation = Orientation.Horizontal };
-            sp.Children.Add(new TextBlock
-            {
-                Text = icon,
-                FontFamily = new FontFamily("Segoe MDL2 Assets"),
-                FontSize = 13,
-                Foreground = iconColor ?? secondaryText,
-                VerticalAlignment = VerticalAlignment.Center,
-                Margin = new Thickness(0, 0, 10, 0),
-            });
-            sp.Children.Add(new TextBlock
-            {
-                Text = label,
-                FontSize = 12.5,
-                Foreground = labelColor ?? primaryText,
-                VerticalAlignment = VerticalAlignment.Center,
-            });
-            var item = new Border
-            {
-                Child = sp,
-                Background = Brushes.Transparent,
-                CornerRadius = new CornerRadius(7),
-                Cursor = Cursors.Hand,
-                Padding = new Thickness(10, 8, 14, 8),
-                Margin = new Thickness(0, 1, 0, 1),
-            };
-            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 += (_, _) =>
+            var item = CreateSurfacePopupMenuItem(icon, iconColor ?? secondaryText, label, () =>
             {
                 popup.IsOpen = false;
                 action();
-            };
+            }, labelColor ?? primaryText);
             panel.Children.Add(item);
         }
 
         void AddSep()
         {
-            panel.Children.Add(new Border
-            {
-                Height = 1,
-                Margin = new Thickness(10, 4, 10, 4),
-                Background = borderBrush,
-                Opacity = 0.3,
-            });
+            panel.Children.Add(CreateSurfacePopupSeparator());
         }
 
         var ext = Path.GetExtension(filePath).ToLowerInvariant();