AX Agent 채팅창 반응형 폭 계산 적용

- ChatWindow의 ComposerShell 고정폭을 제거하고 실제 본문 영역 폭 기준으로 MessagePanel, EmptyState, ComposerShell을 함께 재계산하도록 정리함

- 창 크기 변경 시 메시지 축과 입력창이 자연스럽게 함께 줄고 늘어나도록 Loaded와 SizeChanged에 반응형 레이아웃 갱신을 연결함

- README와 DEVELOPMENT 문서에 2026-04-05 14:16 (KST) 기준 작업 이력을 반영하고 Release 빌드 경고 0 오류 0을 확인함

src/AxCopilot/Views/ChatWindow.xaml.cs

@@ -124,6 +124,8 @@ public partial class ChatWindow : Window
     private readonly Dictionary<string, ChatConversation> _pendingConversationPersists = new(StringComparer.OrdinalIgnoreCase);
     private readonly HashSet<string> _expandedDraftQueueTabs = new(StringComparer.OrdinalIgnoreCase);
     private AgentEvent? _pendingAgentUiEvent;
+    private double _lastResponsiveComposerWidth;
+    private double _lastResponsiveMessageWidth;
     private void ApplyQuickActionVisual(Button button, bool active, string activeBg, string activeFg)
     {
         if (button?.Content is not string text)
@@ -288,6 +290,7 @@ public partial class ChatWindow : Window
             UpdateSidebarModeMenu();
             RefreshContextUsageVisual();
             UpdateTopicPresetScrollMode();
+            UpdateResponsiveChatLayout();
             UpdateInputBoxHeight();
             InputBox.Focus();
             MessageScroll.ScrollChanged += MessageScroll_ScrollChanged;
@@ -299,6 +302,7 @@ public partial class ChatWindow : Window
                 BuildTopicButtons();
                 RestoreLastConversations();
                 RefreshConversationList();
+                UpdateResponsiveChatLayout();
                 UpdateTaskSummaryIndicators();
                 ScheduleGitBranchRefresh();
 
@@ -369,7 +373,12 @@ public partial class ChatWindow : Window
             ApplyHoverScaleAnimation(BtnSend, 1.12);
             ApplyHoverScaleAnimation(BtnStop, 1.12);
         };
-        SizeChanged += (_, _) => UpdateTopicPresetScrollMode();
+        SizeChanged += (_, _) =>
+        {
+            UpdateTopicPresetScrollMode();
+            if (UpdateResponsiveChatLayout())
+                RenderMessages(preserveViewport: true);
+        };
         Closed += (_, _) =>
         {
             _settings.SettingsChanged -= Settings_SettingsChanged;
@@ -11045,15 +11054,61 @@ public partial class ChatWindow : Window
     /// <summary>채팅 본문 폭을 세 탭에서 동일한 기준으로 맞춥니다.</summary>
     private double GetMessageMaxWidth()
     {
-        var hostWidth = ComposerShell?.ActualWidth ?? 0;
+        var hostWidth = _lastResponsiveComposerWidth;
+        if (hostWidth < 100)
+            hostWidth = ComposerShell?.ActualWidth ?? 0;
         if (hostWidth < 100)
             hostWidth = MessageScroll?.ActualWidth ?? 0;
         if (hostWidth < 100)
-            hostWidth = 1120; // 초기화 전 기준 폭
+            hostWidth = 1120;
 
-        // 컴포저와 메시지 버블이 같은 레이아웃 축을 쓰도록 여백만 제외한 폭을 공통 적용
         var maxW = hostWidth - 44;
-        return Math.Clamp(maxW, 320, 720);
+        return Math.Clamp(maxW, 320, 760);
+    }
+
+    private bool UpdateResponsiveChatLayout()
+    {
+        var viewportWidth = MessageScroll?.ActualWidth ?? 0;
+        if (viewportWidth < 200)
+            viewportWidth = ActualWidth;
+        if (viewportWidth < 200)
+            return false;
+
+        // claw-code처럼 창이 줄어들면 메시지 축과 컴포저가 함께 자연스럽게 줄어들도록,
+        // 남는 본문 폭을 기준으로 상한만 두고 반응형 폭을 다시 계산합니다.
+        var contentWidth = Math.Max(360, viewportWidth - 48);
+        var messageWidth = Math.Clamp(contentWidth * 0.9, 360, 920);
+        var composerWidth = Math.Clamp(contentWidth * 0.82, 360, 820);
+
+        if (contentWidth < 760)
+        {
+            messageWidth = Math.Clamp(contentWidth - 8, 340, 820);
+            composerWidth = Math.Clamp(contentWidth - 28, 332, 760);
+        }
+
+        var changed = false;
+        if (Math.Abs(_lastResponsiveMessageWidth - messageWidth) > 1)
+        {
+            _lastResponsiveMessageWidth = messageWidth;
+            if (MessagePanel != null)
+                MessagePanel.MaxWidth = messageWidth;
+            if (EmptyState != null)
+                EmptyState.MaxWidth = messageWidth;
+            changed = true;
+        }
+
+        if (Math.Abs(_lastResponsiveComposerWidth - composerWidth) > 1)
+        {
+            _lastResponsiveComposerWidth = composerWidth;
+            if (ComposerShell != null)
+            {
+                ComposerShell.Width = composerWidth;
+                ComposerShell.MaxWidth = composerWidth;
+            }
+            changed = true;
+        }
+
+        return changed;
     }
 
     private StackPanel CreateStreamingContainer(out TextBlock streamText)