fix: use source fields.length for field count on detail page (not local state)

Agent-Logs-Url: https://github.com/objectstack-ai/objectui/sessions/d101c93c-eeee-40ac-9495-b85336c1f08f

Co-authored-by: hotlong <50353452+hotlong@users.noreply.github.com>

Author: Copilot

apps/console/src/pages/system/ObjectManagerPage.tsx

@@ -216,7 +216,7 @@ function ObjectDetailView({ object, metadataObject, onBack }: ObjectDetailViewPr
           </div>
           <div className="flex items-center gap-2">
             <span className="text-muted-foreground">Fields</span>
-            <Badge variant="outline">{object.fieldCount ?? displayFields.length}</Badge>
+            <Badge variant="outline">{object.fieldCount ?? fields.length}</Badge>
           </div>
           {object.isSystem && (
             <div>