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/machine/MachineCustomFieldsCard.vue

@@ -25,7 +25,7 @@
               <span class="label-text text-sm">{{ field.name }}</span>
             </label>
             <div class="input input-bordered input-sm bg-base-200">
-              {{ formatCustomFieldValue(field) }}
+              {{ formatValueForDisplay(field) }}
             </div>
           </div>
         </div>
@@ -180,7 +180,7 @@
 
 <script setup lang="ts">
 import IconLucideTrash from '~icons/lucide/trash'
-import { formatCustomFieldValue } from '~/shared/utils/customFieldUtils'
+import { formatValueForDisplay } from '~/shared/utils/customFields'
 
 defineProps<{
   customFields: any[]

frontend/app/components/machine/MachineInfoCard.vue

@@ -153,7 +153,7 @@
             </template>
             <template v-else>
               <div class="input input-bordered input-sm bg-base-200">
-                {{ formatCustomFieldValue(field) }}
+                {{ formatValueForDisplay(field) }}
               </div>
             </template>
           </div>
@@ -182,7 +182,7 @@ import { watch } from 'vue'
 import ConstructeurSelect from '~/components/ConstructeurSelect.vue'
 import ConstructeurLinksTable from '~/components/ConstructeurLinksTable.vue'
 import MachineCustomFieldDefEditor from '~/components/machine/MachineCustomFieldDefEditor.vue'
-import { formatCustomFieldValue } from '~/shared/utils/customFieldUtils'
+import { formatValueForDisplay } from '~/shared/utils/customFields'
 import { useMachineCustomFieldDefs } from '~/composables/useMachineCustomFieldDefs'
 import type { ConstructeurLinkEntry } from '~/shared/constructeurUtils'