feat(machine) : add custom field management on machine detail page

- Fix: return customFieldValues in structure endpoint (was hardcoded null)
- Frontend: add editor to create/edit/delete custom field definitions
- Tests: add integration tests for structure values + definition CRUD

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

src/Controller/MachineStructureController.php

@@ -573,7 +573,7 @@ class MachineStructureController extends AbstractController
             'constructeurs'     => $this->normalizeConstructeurs($machine->getConstructeurs()),
             'customFields'      => $this->normalizeCustomFields($machine->getCustomFields()),
             'documents'         => null,
-            'customFieldValues' => null,
+            'customFieldValues' => $this->normalizeCustomFieldValues($machine->getCustomFieldValues()),
         ];
     }