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

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

app/composables/usePieceStructureEditorLogic.ts

@@ -81,6 +81,12 @@ const toEditorField = (
     type: baseType as PieceModelCustomFieldType,
     required: Boolean(input?.required),
     optionsText,
+    defaultValue:
+      input?.defaultValue !== undefined && input.defaultValue !== null && input.defaultValue !== ''
+        ? String(input.defaultValue)
+        : null,
+    ...(typeof input?.id === 'string' && input.id ? { id: input.id } : {}),
+    ...(typeof input?.customFieldId === 'string' && input.customFieldId ? { customFieldId: input.customFieldId } : {}),
     orderIndex: typeof input?.orderIndex === 'number' ? input.orderIndex : index,
   }
 }
@@ -164,6 +170,9 @@ const buildPayload = (
       if (field.customFieldId) {
         payload.customFieldId = field.customFieldId
       }
+      if (field.defaultValue !== undefined && field.defaultValue !== null && field.defaultValue !== '') {
+        payload.defaultValue = String(field.defaultValue)
+      }
 
       if (type === 'select') {
         const options = normalizeLineEndings(field.optionsText)