fix(dashboard): update header border to use colorBorder token

Changed border-bottom color from colorSplit to colorBorder for better theme consistency.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

Author: eschutho

superset-frontend/src/dashboard/components/Header/index.jsx

@@ -100,7 +100,7 @@ import { useHeaderActionsMenu } from './useHeaderActionsDropdownMenu';
 const extensionsRegistry = getExtensionsRegistry();
 
 const headerContainerStyle = theme => css`
-  border-bottom: 1px solid ${theme.colorSplit};
+  border-bottom: 1px solid ${theme.colorBorder};
 `;
 
 const editButtonStyle = theme => css`