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/create.vue

@@ -177,7 +177,6 @@ import DocumentUpload from '~/components/DocumentUpload.vue'
 import { useProductTypes } from '~/composables/useProductTypes'
 import { useProducts } from '~/composables/useProducts'
 import { useToast } from '~/composables/useToast'
-import { useCustomFields } from '~/composables/useCustomFields'
 import { useDocuments } from '~/composables/useDocuments'
 import { formatProductStructurePreview, normalizeProductStructureForSave } from '~/shared/modelUtils'
 import { constructeurIdsFromLinks } from '~/shared/constructeurUtils'
@@ -186,10 +185,7 @@ import { useConstructeurLinks } from '~/composables/useConstructeurLinks'
 import { useConstructeurs } from '~/composables/useConstructeurs'
 import type { ProductModelStructure } from '~/shared/types/inventory'
 import type { ModelType } from '~/services/modelTypes'
-import {
-  type CustomFieldInput,
-  normalizeCustomFieldInputs as normalizeCustomFieldInputsFromUtils,
-} from '~/shared/utils/customFieldFormUtils'
+import { useCustomFieldInputs, type CustomFieldEntityType } from '~/composables/useCustomFieldInputs'
 
 interface ProductCatalogType extends ModelType {
   structure: ProductModelStructure | null
@@ -202,7 +198,6 @@ const router = useRouter()
 const { productTypes, loadProductTypes, loadingProductTypes: loadingTypes } = useProductTypes()
 const { createProduct } = useProducts()
 const toast = useToast()
-const { upsertCustomFieldValue } = useCustomFields()
 const { uploadDocuments } = useDocuments()
 const { canEdit } = usePermissions()
 const { syncLinks } = useConstructeurLinks()
@@ -221,7 +216,14 @@ const constructeurLinks = ref<ConstructeurLinkEntry[]>([])
 const selectedDocuments = ref<File[]>([])
 const uploadingDocuments = ref(false)
 
-const customFieldInputs = ref<CustomFieldInput[]>([])
+const cfDefinitions = ref<any[]>([])
+const createdEntityId = ref<string | null>(null)
+const { fields: customFieldInputs, requiredFilled: requiredCustomFieldsFilled, saveAll: saveAllCustomFields } = useCustomFieldInputs({
+  definitions: cfDefinitions,
+  values: [] as any[],
+  entityType: 'product' as CustomFieldEntityType,
+  entityId: createdEntityId,
+})
 
 const productTypeList = computed<ProductCatalogType[]>(() =>
   (productTypes.value || []) as ProductCatalogType[],
@@ -264,26 +266,17 @@ watch(selectedTypeId, (id) => {
 watch(selectedType, (type) => {
   if (!type) {
     clearForm()
-    customFieldInputs.value = []
+    cfDefinitions.value = []
     return
   }
   if (!creationForm.name) {
     creationForm.name = type.name
   }
-  customFieldInputs.value = normalizeCustomFieldInputsFromUtils(normalizeProductStructureForSave(type.structure))
+  const normalized = normalizeProductStructureForSave(type.structure)
+  cfDefinitions.value = normalized?.customFields ?? []
 })
 
-const requiredCustomFieldsFilled = computed(() =>
-  customFieldInputs.value.every((field) => {
-    if (!field.required) {
-      return true
-    }
-    if (field.type === 'boolean') {
-      return field.value === 'true' || field.value === 'false'
-    }
-    return field.value.trim().length > 0
-  }),
-)
+// requiredCustomFieldsFilled comes from useCustomFieldInputs composable
 
 const canSubmit = computed(() => Boolean(
   canEdit.value &&
@@ -336,7 +329,8 @@ const submitCreation = async () => {
     const result = await createProduct(payload)
     if (result.success && result.data?.id) {
       const productId = result.data.id
-      const failedFields = await saveCustomFieldValues(result.data.id)
+      createdEntityId.value = productId
+      const failedFields = await saveAllCustomFields()
       if (failedFields.length) {
         toast.showError(`Produit créé, mais impossible d'enregistrer ${failedFields.length} champ(s): ${failedFields.join(', ')}`)
         await router.replace(`/product/${result.data.id}?edit=true`)
@@ -375,39 +369,6 @@ const submitCreation = async () => {
   }
 }
 
-const saveCustomFieldValues = async (productId: string) => {
-  const failed: string[] = []
-  for (const field of customFieldInputs.value) {
-    if (!field.name) {
-      continue
-    }
-    const value = field.value ?? ''
-    const metadata = field.customFieldId
-      ? undefined
-      : { customFieldName: field.name, customFieldType: field.type, customFieldRequired: field.required }
-    const result = await upsertCustomFieldValue(
-      field.customFieldId,
-      'product',
-      productId,
-      String(value ?? ''),
-      metadata,
-    )
-    if (!result.success) {
-      failed.push(field.name)
-    } else {
-      const createdValue = result.data
-      if (createdValue?.id) {
-        field.customFieldValueId = createdValue.id
-      }
-      const resolvedId = createdValue?.customField?.id || field.customFieldId
-      if (resolvedId) {
-        field.customFieldId = resolvedId
-      }
-    }
-  }
-  return failed
-}
-
 // Sync constructeurIds → constructeurLinks when IDs are added via ConstructeurSelect
 watch(
   () => creationForm.constructeurIds,