feat(ui): populate material combobox from post processors

Replaces the material textbox on EditNestInfoForm with a combobox whose
items are aggregated from every loaded post processor that implements the
new IMaterialProvidingPostProcessor interface. CincinnatiPostProcessor
exposes its configured MaterialLibraries entries. Free-text entry still
works so custom materials remain usable.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest.Core/IMaterialProvidingPostProcessor.cs

@@ -0,0 +1,9 @@
+using System.Collections.Generic;
+
+namespace OpenNest
+{
+    public interface IMaterialProvidingPostProcessor
+    {
+        IEnumerable<string> GetMaterialNames();
+    }
+}