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/create.vue

@@ -813,13 +813,14 @@ const fetchPieceTypeNames = async (ids: string[]) => {
   )
   const next = { ...fetchedPieceTypeMap.value }
   results.forEach((result, index) => {
-    if (result.status !== 'fulfilled') {
+    const key = missing[index]
+    if (!key || result.status !== 'fulfilled') {
       return
     }
     const data = result.value?.data
     const name = data?.name || data?.code
     if (name) {
-      next[missing[index]] = name
+      next[key] = name
     }
   })
   fetchedPieceTypeMap.value = next
@@ -1144,7 +1145,7 @@ const resolveOptions = (field: any): string[] => {
         return option.trim()
       }
       if (typeof option === 'object') {
-        const record = option || {}
+        const record = option as Record<string, unknown>
         const keys = ['value', 'label', 'name']
         for (const key of keys) {
           const candidate = record[key]