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/shared/model/componentStructure.ts

@@ -4,6 +4,7 @@ import {
   type ComponentModelStructure,
   type ComponentModelStructureNode,
 } from '../types/inventory'
+import { mergeDefinitionsWithValues } from '../utils/customFields'
 
 // Import for internal use in this file
 import { sanitizeCustomFields, sanitizePieces, sanitizeProducts, sanitizeSubcomponents } from './componentStructureSanitize'
@@ -86,30 +87,22 @@ export const cloneStructure = (input: any): ComponentModelStructure => {
 export const normalizeStructureForEditor = (input: any): ComponentModelStructure => {
   const source = cloneStructure(input)
 
-  const sanitizedCustomFields = sanitizeCustomFields(source.customFields)
-  const customFields = sanitizedCustomFields.map((field) => {
-    const options = Array.isArray(field.options) ? [...field.options] : []
-    const optionsText = options.length ? options.join('\n') : ''
-    const defaultValue =
-      field.defaultValue !== undefined && field.defaultValue !== null && field.defaultValue !== ''
-        ? String(field.defaultValue)
-        : null
-    const copy: ComponentModelCustomField = {
-      name: field.name,
-      type: field.type,
-      required: field.required,
-      machineContextOnly: !!field.machineContextOnly,
-      options,
-      defaultValue,
-      optionsText,
-      id: field.id,
-      customFieldId: field.customFieldId,
-    }
-    return copy
-  })
+  const merged = mergeDefinitionsWithValues(source.customFields, [])
+  const customFields: ComponentModelCustomField[] = merged.map((field) => ({
+    name: field.name,
+    type: field.type as ComponentModelCustomField['type'],
+    required: field.required,
+    machineContextOnly: field.machineContextOnly,
+    options: field.options,
+    defaultValue: field.defaultValue,
+    optionsText: field.optionsText,
+    id: field.customFieldId ?? undefined,
+    customFieldId: field.customFieldId ?? undefined,
+    orderIndex: field.orderIndex,
+  }))
 
   const result: ComponentModelStructure = {
-    customFields: customFields as ComponentModelCustomField[],
+    customFields,
     pieces: sanitizePieces(source.pieces),
     products: sanitizeProducts(source.products),
     subcomponents: hydrateSubcomponents(source.subcomponents),