Group create-node list by source and category; make editor columns resizable

The create-node list is now a two-level tree: a header per node source
(USD, MaterialX, Arnold, Cycles - from the Sdr sourceType, with glslfx
and OSL merged into USD so core nodes don't appear twice) containing
Hypershade-style category sub-trees (Material / Texture / Geometry /
Utility / Light) derived from Sdr context, terminal output types, role,
and name keywords. Duplicate collapsing is per (source, label) so
same-named nodes from different providers both stay; searching forces
all sections open. The TAB popup shows [Source/Category] per entry.

The browser and preview columns get drag splitters (viewport-divider
styling, clamped widths); the canvas absorbs the remainder.

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

src/ui/MaterialEditorPanel.h

@@ -133,6 +133,11 @@ private:
     char m_nodeSearchBuf[128] = "";
     int  m_nodeSearchSelected = 0;
 
+    /// Column widths of the browser (left) and preview (right) sections,
+    /// user-adjustable via the splitters between them and the canvas.
+    float m_browserWidth = 220.0f;
+    float m_previewWidth = 320.0f;
+
     /// Search text of the browser's persistent create-node list.
     char m_browserNodeSearchBuf[128] = "";
     /// Shader id clicked in the browser's create-node list; created at the