refactor: repartition server-side and client-side code

app/.client/components/ui/PanelHeader.tsx

@@ -0,0 +1,20 @@
+import classNames from 'classnames';
+import { memo } from 'react';
+
+interface PanelHeaderProps {
+  className?: string;
+  children: React.ReactNode;
+}
+
+export const PanelHeader = memo(({ className, children }: PanelHeaderProps) => {
+  return (
+    <div
+      className={classNames(
+        'flex items-center gap-2 bg-upage-elements-background-depth-2 text-upage-elements-textSecondary border-b border-upage-elements-borderColor px-4 py-1 min-h-[34px] text-sm',
+        className,
+      )}
+    >
+      {children}
+    </div>
+  );
+});