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/components/ComponentModelStructureEditor.vue

@@ -10,6 +10,7 @@
       :locked-type-label="displayedRootTypeLabel"
       :allow-subcomponents="allowSubcomponents"
       :max-subcomponent-depth="maxSubcomponentDepth"
+      :restricted-mode="restrictedMode"
       is-root
     />
   </div>
@@ -55,6 +56,10 @@ const props = defineProps({
     type: Number,
     default: Infinity,
   },
+  restrictedMode: {
+    type: Boolean,
+    default: false,
+  },
 })
 
 const emit = defineEmits(['update:modelValue'])