fix(custom-fields) : preserve CustomField ID in piece structure payload

Prevents data loss when saving ModelType: the frontend now sends existing
CustomField IDs so the backend can match them instead of deleting and recreating.

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

app/composables/usePieceStructureEditorLogic.ts

@@ -158,6 +158,13 @@ const buildPayload = (
         orderIndex: index,
       }
 
+      if (field.id) {
+        payload.id = field.id
+      }
+      if (field.customFieldId) {
+        payload.customFieldId = field.customFieldId
+      }
+
       if (type === 'select') {
         const options = normalizeLineEndings(field.optionsText)
           .split('\n')