refactor(components): extract shared entity utilities and simplify item components (F1.3, F1.4)

Extract 3 entity composables (useEntityCustomFields, useEntityDocuments,
useEntityProductDisplay) and entityCustomFieldLogic utility shared across
ComponentItem (1336→585 LOC) and PieceItem (1588→740 LOC).
Improve type safety in edit/create pages with explicit casts.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/pages/component-category/[id]/edit.vue

@@ -42,6 +42,7 @@ import { computed, onMounted, ref } from 'vue'
 import { useHead, useRoute, useRouter } from '#imports'
 import ModelTypeForm from '~/components/model-types/ModelTypeForm.vue'
 import { getModelType, updateModelType, type ModelTypePayload } from '~/services/modelTypes'
+import type { ComponentModelStructure } from '~/shared/types/inventory'
 import { useCategoryEditGuard } from '~/composables/useCategoryEditGuard'
 import { useToast } from '~/composables/useToast'
 
@@ -108,7 +109,7 @@ const loadCategory = async () => {
       code: response.code,
       category: response.category,
       notes: response.notes ?? response.description ?? '',
-      structure: response.structure ?? undefined,
+      structure: (response.structure as ComponentModelStructure | null) ?? undefined,
     }
 
     await loadLinkedCount(id)