Persist material editor column widths in preferences

MaterialBrowserWidth / MaterialPreviewWidth in preferences.ini, saved
from the live splitter positions on shutdown and restored (clamped to
the splitter ranges) at startup.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/ui/Application.h

@@ -28,6 +28,8 @@ struct AppPreferences {
     std::string ocioView;
     std::string ocioColorSpace;
     std::string ocioLook;
+    float materialBrowserWidth = 220.0f; ///< Material Editor left-column width
+    float materialPreviewWidth = 320.0f; ///< Material Editor right-column width
 };
 
 class Application {