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/components/PieceItem.vue

@@ -319,16 +319,10 @@ import {
   uniqueConstructeurIds,
   parseConstructeurLinksFromApi,
 } from '~/shared/constructeurUtils'
-import {
-  resolveFieldId,
-  resolveFieldReadOnly,
-  resolveCustomFieldId,
-  mergeFieldDefinitionsWithValues,
-  dedupeMergedFields,
-} from '~/shared/utils/entityCustomFieldLogic'
+import { mergeDefinitionsWithValues } from '~/shared/utils/customFields'
+import { useCustomFields } from '~/composables/useCustomFields'
 import { useEntityDocuments } from '~/composables/useEntityDocuments'
 import { useEntityProductDisplay } from '~/composables/useEntityProductDisplay'
-import { useEntityCustomFields } from '~/composables/useEntityCustomFields'
 
 const props = defineProps({
   piece: { type: Object, required: true },
@@ -392,25 +386,81 @@ const {
 } = useEntityProductDisplay({ entity: () => props.piece, selectedProduct })
 
 const {
-  displayedCustomFields,
-  updateCustomField,
-} = useEntityCustomFields({ entity: () => props.piece, entityType: 'piece' })
+  updateCustomFieldValue: updateCustomFieldValueApi,
+  upsertCustomFieldValue,
+} = useCustomFields()
+const { showSuccess, showError } = useToast()
 
+// Parent already pre-merges standalone custom fields into props.piece.customFields
+const displayedCustomFields = computed(() => {
+  const fields = props.piece?.customFields
+  return Array.isArray(fields) ? fields.filter((f) => !f.machineContextOnly) : []
+})
+
+const updateCustomField = async (field) => {
+  if (!field || field.readOnly) return
+
+  const e = props.piece
+  const fieldValueId = field.customFieldValueId
+
+  if (fieldValueId) {
+    const result = await updateCustomFieldValueApi(fieldValueId, { value: field.value ?? '' })
+    if (result.success) {
+      showSuccess(`Champ "${field.name}" mis à jour avec succès`)
+    } else {
+      showError(`Erreur lors de la mise à jour du champ "${field.name}"`)
+    }
+    return
+  }
+
+  if (!e?.id) {
+    showError('Impossible de créer la valeur pour ce champ')
+    return
+  }
+
+  const metadata = field.customFieldId ? undefined : {
+    customFieldName: field.name,
+    customFieldType: field.type,
+    customFieldRequired: field.required,
+    customFieldOptions: field.options,
+  }
+  const result = await upsertCustomFieldValue(
+    field.customFieldId,
+    'piece',
+    e.id,
+    field.value ?? '',
+    metadata,
+  )
+
+  if (result.success) {
+    const newValue = result.data
+    if (newValue?.id) {
+      field.customFieldValueId = newValue.id
+      field.value = newValue.value ?? field.value ?? ''
+      if (newValue.customField?.id) {
+        field.customFieldId = newValue.customField.id
+      }
+    }
+    showSuccess(`Champ "${field.name}" créé avec succès`)
+  } else {
+    showError(`Erreur lors de la sauvegarde du champ "${field.name}"`)
+  }
+}
+
+// Context fields are NOT pre-merged — merge locally
 const mergedContextFields = computed(() => {
   const definitions = props.piece?.contextCustomFields ?? []
   const values = props.piece?.contextCustomFieldValues ?? []
   if (!definitions.length && !values.length) return []
-  return dedupeMergedFields(
-    mergeFieldDefinitionsWithValues(definitions, values),
-  )
+  return mergeDefinitionsWithValues(definitions, values)
 })
 
 const queueContextCustomFieldUpdate = (field, value) => {
   const linkId = props.piece?.linkId
   if (!linkId || !field) return
 
-  const customFieldId = resolveCustomFieldId(field)
-  const customFieldValueId = resolveFieldId(field)
+  const customFieldId = field.customFieldId
+  const customFieldValueId = field.customFieldValueId
   if (!customFieldId && !customFieldValueId) return
 
   field.value = value
@@ -420,7 +470,7 @@ const queueContextCustomFieldUpdate = (field, value) => {
     fieldId: customFieldId,
     customFieldValueId,
     value: value ?? '',
-    fieldName: field.name || field.customField?.name || 'Champ contextuel',
+    fieldName: field.name || 'Champ contextuel',
   })
 }
 
@@ -544,8 +594,8 @@ const handleProductChange = async (value) => {
 
 // --- Custom field event handlers ---
 const handleCustomFieldInput = (field, value) => {
-  if (resolveFieldReadOnly(field)) return
-  const fieldValueId = resolveFieldId(field)
+  if (field.readOnly) return
+  const fieldValueId = field.customFieldValueId
   if (!fieldValueId) return
   const fieldValue = props.piece.customFieldValues?.find((fv) => fv.id === fieldValueId)
   if (fieldValue) fieldValue.value = value
@@ -553,7 +603,7 @@ const handleCustomFieldInput = (field, value) => {
 
 const handleCustomFieldBlur = async (field) => {
   await updateCustomField(field)
-  const cfId = field?.customFieldId || field?.customField?.id || null
+  const cfId = field?.customFieldId || null
   if (cfId || field?.customFieldValueId) {
     emit('custom-field-update', {
       fieldId: cfId,