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/MaterialEditorPanel.h

@@ -44,6 +44,14 @@ public:
     /// auto-loads it into the canvas.
     void SetTargetPrimPath(const std::string& path);
 
+    /// Splitter-adjustable column widths, persisted in preferences.ini by
+    /// Application. Setter clamps to the same range as the splitters.
+    void GetColumnWidths(float& browserWidth, float& previewWidth) const {
+        browserWidth = m_browserWidth;
+        previewWidth = m_previewWidth;
+    }
+    void SetColumnWidths(float browserWidth, float previewWidth);
+
     void Render();
 
 private: