fix(custom-fields) : preserve defaultValue and IDs in piece structure editor

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

app/shared/model/pieceProductStructure.ts

@@ -86,6 +86,19 @@ const sanitizePieceCustomFields = (fields: any[]): PieceModelCustomField[] => {
       if (options) {
         result.options = options
       }
+      const defaultValue =
+        field?.defaultValue !== undefined && field?.defaultValue !== null && field?.defaultValue !== ''
+          ? String(field.defaultValue)
+          : null
+      if (defaultValue !== null) {
+        result.defaultValue = defaultValue
+      }
+      if (typeof field?.id === 'string' && field.id) {
+        result.id = field.id
+      }
+      if (typeof field?.customFieldId === 'string' && field.customFieldId) {
+        result.customFieldId = field.customFieldId
+      }
       const orderIndex = typeof field?.orderIndex === 'number' ? field.orderIndex : index
       result.orderIndex = orderIndex
       return result