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/pieces/create.vue

@@ -225,7 +225,6 @@ import { usePieceTypes } from '~/composables/usePieceTypes'
 import { usePieces } from '~/composables/usePieces'
 import { useToast } from '~/composables/useToast'
 import { humanizeError } from '~/shared/utils/errorMessages'
-import { useCustomFields } from '~/composables/useCustomFields'
 import { useDocuments } from '~/composables/useDocuments'
 import { useConstructeurLinks } from '~/composables/useConstructeurLinks'
 import { useConstructeurs } from '~/composables/useConstructeurs'
@@ -243,12 +242,7 @@ import {
   applyProductSelection,
   collectNormalizedProductIds,
 } from '~/shared/utils/pieceProductSelectionUtils'
-import {
-  type CustomFieldInput,
-  normalizeCustomFieldInputs,
-  requiredCustomFieldsFilled as _requiredCustomFieldsFilled,
-  saveCustomFieldValues as _saveCustomFieldValues,
-} from '~/shared/utils/customFieldFormUtils'
+import { useCustomFieldInputs, type CustomFieldEntityType } from '~/composables/useCustomFieldInputs'
 
 interface PieceCatalogType extends ModelType {
   structure: PieceModelStructure | null
@@ -261,7 +255,6 @@ const router = useRouter()
 const { pieceTypes, loadPieceTypes, loadingPieceTypes: loadingTypes } = usePieceTypes()
 const { createPiece } = usePieces()
 const toast = useToast()
-const { upsertCustomFieldValue, updateCustomFieldValue } = useCustomFields()
 const { uploadDocuments } = useDocuments()
 const { syncLinks } = useConstructeurLinks()
 const { getConstructeurById } = useConstructeurs()
@@ -281,7 +274,14 @@ const constructeurLinks = ref<ConstructeurLinkEntry[]>([])
 const productSelections = ref<(string | null)[]>([])
 
 const lastSuggestedName = ref('')
-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: 'piece' as CustomFieldEntityType,
+  entityId: createdEntityId,
+})
 const selectedDocuments = ref<File[]>([])
 const uploadingDocuments = ref(false)
 
@@ -360,21 +360,17 @@ watch(structureProducts, (products) => {
 watch(selectedType, (type) => {
   if (!type) {
     clearCreationForm()
-    customFieldInputs.value = []
+    cfDefinitions.value = []
     return
   }
   if (!creationForm.name || creationForm.name === lastSuggestedName.value) {
     creationForm.name = type.name
   }
   lastSuggestedName.value = creationForm.name
-  customFieldInputs.value = normalizeCustomFieldInputs(type.structure)
+  cfDefinitions.value = type.structure?.customFields ?? []
   productSelections.value = Array.from({ length: structureProducts.value.length }, () => null)
 })
 
-const requiredCustomFieldsFilled = computed(() =>
-  _requiredCustomFieldsFilled(customFieldInputs.value),
-)
-
 const canSubmit = computed(() =>
   Boolean(
     canEdit.value &&
@@ -450,14 +446,11 @@ const submitCreation = async () => {
     const result = await createPiece(payload)
     if (result.success && result.data) {
       const createdPiece = result.data as Record<string, any>
-      await _saveCustomFieldValues(
-        'piece',
-        createdPiece.id,
-        [
-          createdPiece?.typePiece?.structure?.customFields,
-        ],
-        { customFieldInputs, upsertCustomFieldValue, updateCustomFieldValue, toast },
-      )
+      createdEntityId.value = createdPiece.id
+      const failedFields = await saveAllCustomFields()
+      if (failedFields.length) {
+        toast.showError(`Pièce créée, mais impossible d'enregistrer ${failedFields.length} champ(s): ${failedFields.join(', ')}`)
+      }
       // Sync constructeur links after creation
       if (constructeurLinks.value.length) {
         await syncLinks('piece', createdPiece.id, [], constructeurLinks.value)