feat(model-types): allow adding custom fields in restricted mode

When a category has linked items (pieces, components, products),
enable restricted mode instead of blocking all edits:
- Allow adding new custom fields
- Lock existing fields from modification or deletion
- Hide add buttons for products, pieces, and subcomponents
- Display informative message about restricted mode

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

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

@@ -28,6 +28,8 @@
         :saving="saving"
         :disable-submit="isSubmitBlocked"
         :disable-submit-message="submitBlockMessage"
+        :restricted-mode="isRestrictedMode"
+        :restricted-mode-message="restrictedModeMessage"
         @submit="handleSubmit"
         @cancel="handleCancel"
       />
@@ -52,7 +54,9 @@ const saving = ref(false)
 const initialData = ref<Partial<ModelTypePayload> | null>(null)
 
 const {
+  isRestrictedMode,
   isSubmitBlocked,
+  restrictedModeMessage,
   submitBlockMessage,
   loadLinkedCount,
   guardSubmitOrNotify,