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.Posts.Cincinnati/CincinnatiPostProcessor.cs

@@ -9,7 +9,7 @@ using OpenNest.CNC;
 
 namespace OpenNest.Posts.Cincinnati
 {
-    public sealed class CincinnatiPostProcessor : IConfigurablePostProcessor
+    public sealed class CincinnatiPostProcessor : IConfigurablePostProcessor, IMaterialProvidingPostProcessor
     {
         private static readonly JsonSerializerOptions JsonOptions = new()
         {
@@ -25,6 +25,16 @@ namespace OpenNest.Posts.Cincinnati
 
         object IConfigurablePostProcessor.Config => Config;
 
+        public IEnumerable<string> GetMaterialNames()
+        {
+            if (Config?.MaterialLibraries == null)
+                return System.Array.Empty<string>();
+
+            return Config.MaterialLibraries
+                .Select(e => e.Material)
+                .Where(s => !string.IsNullOrWhiteSpace(s));
+        }
+
         public CincinnatiPostProcessor()
         {
             var configPath = GetConfigPath();