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/product/[id]/edit.vue

@@ -237,7 +237,6 @@ import ConstructeurSelect from '~/components/ConstructeurSelect.vue'
 import DocumentUpload from '~/components/DocumentUpload.vue'
 import DocumentPreviewModal from '~/components/DocumentPreviewModal.vue'
 import { useProducts } from '~/composables/useProducts'
-import { useCustomFields } from '~/composables/useCustomFields'
 import { useToast } from '~/composables/useToast'
 import { humanizeError } from '~/shared/utils/errorMessages'
 import { useDocuments } from '~/composables/useDocuments'
@@ -250,12 +249,7 @@ import type { ConstructeurLinkEntry } from '~/shared/constructeurUtils'
 import { getModelType } from '~/services/modelTypes'
 import type { ProductModelStructure } from '~/shared/types/inventory'
 import { canPreviewDocument } from '~/utils/documentPreview'
-import {
-  type CustomFieldInput,
-  buildCustomFieldInputs,
-  requiredCustomFieldsFilled as _requiredCustomFieldsFilled,
-  saveCustomFieldValues as _saveCustomFieldValues,
-} from '~/shared/utils/customFieldFormUtils'
+import { useCustomFieldInputs, type CustomFieldEntityType } from '~/composables/useCustomFieldInputs'
 
 const { canEdit } = usePermissions()
 const versionRefreshKey = ref(0)
@@ -263,7 +257,6 @@ const route = useRoute()
 const router = useRouter()
 const toast = useToast()
 const { getProduct, updateProduct } = useProducts()
-const { upsertCustomFieldValue, updateCustomFieldValue } = useCustomFields()
 const {
   loadDocumentsByProduct,
   uploadDocuments: uploadProductDocuments,
@@ -282,7 +275,19 @@ const {
 const product = ref<any | null>(null)
 const productType = ref<any | null>(null)
 const structure = ref<ProductModelStructure | null>(null)
-const customFieldInputs = ref<CustomFieldInput[]>([])
+const cfDefinitions = ref<any[]>([])
+const cfValues = ref<any[]>([])
+const entityId = computed(() => product.value?.id ?? null)
+const {
+  fields: customFieldInputs,
+  requiredFilled: requiredCustomFieldsFilled,
+  saveAll: saveAllCustomFields,
+} = useCustomFieldInputs({
+  definitions: cfDefinitions,
+  values: cfValues,
+  entityType: 'product' as CustomFieldEntityType,
+  entityId,
+})
 const loading = ref(true)
 const saving = ref(false)
 const selectedFiles = ref<File[]>([])
@@ -311,7 +316,8 @@ const refreshCustomFieldInputs = (
 ) => {
   const nextStructure = structureOverride ?? structure.value ?? null
   const nextValues = valuesOverride ?? product.value?.customFieldValues ?? null
-  customFieldInputs.value = buildCustomFieldInputs(nextStructure, nextValues)
+  cfDefinitions.value = nextStructure?.customFields ?? []
+  cfValues.value = Array.isArray(nextValues) ? nextValues : []
 }
 
 const editionForm = reactive({
@@ -321,9 +327,7 @@ const editionForm = reactive({
   supplierPrice: '' as string,
 })
 
-const requiredCustomFieldsFilled = computed(() =>
-  _requiredCustomFieldsFilled(customFieldInputs.value),
-)
+// requiredCustomFieldsFilled comes from useCustomFieldInputs composable
 
 const canSubmit = computed(() =>
   Boolean(canEdit.value && product.value && editionForm.name.trim().length >= 2 && requiredCustomFieldsFilled.value && !saving.value),
@@ -517,12 +521,7 @@ const submitEdition = async () => {
     const result = await updateProduct(product.value.id, payload)
     if (result.success && result.data?.id) {
       product.value = result.data
-      const failedFields = await _saveCustomFieldValues(
-        'product',
-        result.data.id,
-        [result.data?.typeProduct?.structure?.customFields],
-        { customFieldInputs, upsertCustomFieldValue, updateCustomFieldValue, toast },
-      )
+      const failedFields = await saveAllCustomFields()
       if (failedFields.length) {
         toast.showError(`Impossible d'enregistrer ${failedFields.length} champ(s): ${failedFields.join(', ')}`)
         return