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

docs/DEVELOPMENT.md

@@ -4935,3 +4935,5 @@ ow + toggle ?쒓컖 ?몄뼱濡??ㅼ떆 ?뺣젹?덈떎.
 - Document update: 2026-04-06 11:03 (KST) - This leaves the main chat window even more orchestration-focused and effectively closes the last obvious sidebar interaction block from the large structure-improvement plan. Remaining work is now follow-up UX polish rather than major decomposition.
 - Document update: 2026-04-06 11:11 (KST) - Split conversation list filter/sort interactions and preference persistence out of `ChatWindow.xaml.cs` into `ChatWindow.ConversationFilterPresentation.cs`. Running-only toggles, recent/activity sort toggles, UI-state refresh, and conversation-list preference apply/persist logic now live in a dedicated presentation partial.
 - Document update: 2026-04-06 11:11 (KST) - This keeps sidebar state presentation more cohesive and further narrows the main chat window file toward orchestration-only responsibilities. Remaining work is now post-plan polish rather than another major structural split.
+- Document update: 2026-04-06 11:20 (KST) - Added `ChatWindow.SurfaceVisualPresentation.cs` and introduced shared surface helpers for popup containers, popup menu rows, separators, and file-tree headers. `ChatWindow.PreviewPresentation.cs` and `ChatWindow.FileBrowserPresentation.cs` now use the same popup/surface language instead of duplicating slightly different visual styles.
+- Document update: 2026-04-06 11:20 (KST) - This is the first visual-language polish pass after the large structure split. It reduces the feeling that preview and file browser are separate widgets and makes future popup/surface refinements reusable.