refactor: wire ExportDXF to use FabWorks API

Replace direct DB access with API client calls throughout MainForm,
DxfExportService, PartExporter, and Program. Add title field to UI,
async export flow, API-based dropdown loading, and file uploads.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ExportDXF/Models/Item.cs

@@ -68,8 +68,9 @@ namespace ExportDXF.Services
         public string ContentHash { get; set; }
 
         /// <summary>
-        /// Path to the stashed (backed-up) previous DXF file (transient, not persisted).
+        /// Full path to the locally-exported DXF temp file (transient, not persisted).
+        /// Set after successful export; used for upload to the API.
         /// </summary>
-        public string StashedFilePath { get; set; }
+        public string LocalTempPath { get; set; }
     }
 }