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/components/PieceModelStructureEditor.vue

@@ -511,6 +511,10 @@ const reorderFields = (from: number, to: number) => {
   }
 
   const [moved] = list.splice(from, 1)
+  if (!moved) {
+    resetDragState()
+    return
+  }
   list.splice(to, 0, moved)
   fields.value = applyOrderIndex(list)
   resetDragState()