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.cpp

@@ -148,6 +148,11 @@ std::vector<const ShaderNodeTypeInfo*> FilterShaderNodeTypes(
     return matches;
 }
 
+// Clamp range shared by the column splitters and SetColumnWidths (persisted
+// preferences may carry hand-edited or stale values).
+constexpr float kBrowserMinWidth = 140.0f, kBrowserMaxWidth = 500.0f;
+constexpr float kPreviewMinWidth = 220.0f, kPreviewMaxWidth = 640.0f;
+
 // Vertical drag-splitter between the material editor's columns. Adjusts
 // *width by the mouse drag (negated for a right-hand column, which grows
 // when dragged left); colours match the viewport's split dividers.
@@ -238,6 +243,11 @@ void MaterialEditorPanel::SetStage(pxr::UsdStageRefPtr stage) {
     m_browserSelection = pxr::SdfPath();
 }
 
+void MaterialEditorPanel::SetColumnWidths(float browserWidth, float previewWidth) {
+    m_browserWidth = std::clamp(browserWidth, kBrowserMinWidth, kBrowserMaxWidth);
+    m_previewWidth = std::clamp(previewWidth, kPreviewMinWidth, kPreviewMaxWidth);
+}
+
 void MaterialEditorPanel::SetColorCorrectionFromPrefs(int ccMode, const std::string& ocioDisplay,
                                                       const std::string& ocioView,
                                                       const std::string& ocioColorSpace,
@@ -261,14 +271,14 @@ void MaterialEditorPanel::Render() {
     RenderMaterialBrowser();
     ImGui::EndChild();
 
-    VerticalSplitter("##MaterialSplitL", &m_browserWidth, 140.0f, 500.0f,
+    VerticalSplitter("##MaterialSplitL", &m_browserWidth, kBrowserMinWidth, kBrowserMaxWidth,
                      /*rightSideColumn=*/false);
 
     ImGui::BeginChild("MaterialCanvasRegion", ImVec2(-(m_previewWidth + 6.0f), 0.0f), false);
     RenderNodeGraphCanvas();
     ImGui::EndChild();
 
-    VerticalSplitter("##MaterialSplitR", &m_previewWidth, 220.0f, 640.0f,
+    VerticalSplitter("##MaterialSplitR", &m_previewWidth, kPreviewMinWidth, kPreviewMaxWidth,
                      /*rightSideColumn=*/true);
 
     ImGui::BeginChild("MaterialPreviewRegion", ImVec2(0.0f, 0.0f), true);