fix(custom-fields) : use structure.customFields for definition lookup

The definitionSources passed to saveCustomFieldValues were pointing at
properties not serialized by the API (typeComposant.customFields,
typePiece.pieceCustomFields). Changed to structure.customFields which
is the correct serialized path, preventing orphan custom field creation.

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

app/composables/useComponentCreate.ts

@@ -323,7 +323,7 @@ export function useComponentCreate() {
         await _saveCustomFieldValues(
           'composant',
           createdComponent.id,
-          [createdComponent?.typeComposant?.customFields],
+          [createdComponent?.typeComposant?.structure?.customFields],
           { customFieldInputs, upsertCustomFieldValue, updateCustomFieldValue, toast },
         )
         if (selectedDocuments.value.length && result.data?.id) {