refactor(custom-fields) : unify 3 parallel implementations into 1 module

Replace ~2900 lines across 9 files with ~400 lines in 2 files:
- shared/utils/customFields.ts (types + pure helpers)
- composables/useCustomFieldInputs.ts (reactive composable)

Migrated all consumers:
- Backend: add defaultValue to API Platform serialization groups
- Standalone pages: component edit/create, piece edit/create, product edit/create/detail
- Machine page: MachineCustomFieldsCard, MachineInfoCard, useMachineDetailCustomFields
- Hierarchy: ComponentItem, PieceItem
- Shared: CustomFieldDisplay, CustomFieldInputGrid
- Category editor: componentStructure.ts

Deleted:
- entityCustomFieldLogic.ts (335 lines)
- customFieldUtils.ts (440 lines)
- customFieldFormUtils.ts (404 lines)
- useEntityCustomFields.ts (181 lines)
- customFieldFormUtils.test.ts

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

frontend/app/pages/component/[id]/index.vue

@@ -336,7 +336,7 @@
               <div class="grid grid-cols-1 gap-4 md:grid-cols-2">
                 <div
                   v-for="field in visibleCustomFields"
-                  :key="field.customFieldValueId || field.id || field.name"
+                  :key="field.customFieldValueId || field.customFieldId || field.name"
                   class="form-control"
                 >
                   <label class="label">