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/pieces/[id]/edit.vue

@@ -503,7 +503,6 @@ import {
 import {
   historyActionLabel,
   formatHistoryDate,
-  formatHistoryValue,
   historyDiffEntries as _historyDiffEntries,
 } from '~/shared/utils/historyDisplayUtils'
 
@@ -626,7 +625,7 @@ const refreshDocuments = async () => {
   try {
     const result = await loadDocumentsByPiece(piece.value.id, { updateStore: false })
     if (result.success) {
-      pieceDocuments.value = result.data || []
+      pieceDocuments.value = Array.isArray(result.data) ? result.data : result.data ? [result.data] : []
     }
   } finally {
     loadingDocuments.value = false
@@ -776,9 +775,9 @@ const loadPieceTypeDetails = async (currentPiece: any) => {
     const type = await getModelType(typeId)
     if (type && typeof type === 'object') {
       pieceTypeDetails.value = type
-      refreshCustomFieldInputs(type.structure ?? null, currentPiece?.customFieldValues ?? null)
+      refreshCustomFieldInputs((type.structure as PieceModelStructure | null) ?? null, currentPiece?.customFieldValues ?? null)
     }
-  } catch (error) {
+  } catch (_error) {
     pieceTypeDetails.value = null
   }
 }
@@ -898,8 +897,8 @@ const submitEdition = async () => {
   saving.value = true
   try {
     const result = await updatePiece(piece.value.id, payload)
-    if (result.success) {
-      const updatedPiece = result.data
+    if (result.success && result.data) {
+      const updatedPiece = result.data as Record<string, any>
       await _saveCustomFieldValues(
         'piece',
         updatedPiece.id,