Remove WorkflowPage and associated references from AppShell and Sidebar components

frontend/src/components/pages/WorkflowPage.tsx

@@ -1,10 +0,0 @@
-import { PlaceholderPage } from "./PlaceholderPage";
-
-export function WorkflowPage() {
-  return (
-    <PlaceholderPage
-      title="工作流"
-      description="管理与编排可复用的助手工作流，支持多轮任务与工具调用。"
-    />
-  );
-}