[JKobrynski]

Summary

Fixes a duplicated mobile sidebar menu icon that was reported on Slack.

In newer Rspress versions, the mobile sidebar Menu button renders its built-in icon as an inline svg. The theme already adds its own menu icon via ::before, but only hid the old img icon shape. As a result, both icons could appear next to each other on mobile.

This updates the CSS rule to hide both img and svg children inside .rp-sidebar-menu__left, leaving only the theme-provided icon visible.

Test plan

• Open the tester app on a mobile viewport.
• Visit a sidebar page, for example /nav/section-a/section-a-intro.
• Verify the sticky Menu row shows only one menu icon.
• Tap Menu and verify the sidebar still opens correctly.
• Visit /test/guide on mobile and verify:
  • Menu still opens the sidebar.
  • Contents still opens the outline panel.
  • The top mobile nav hamburger still opens the nav menu.
• Switch to desktop width and verify:
  • The regular sidebar is visible.
  • The mobile sticky menu row is collapsed/hidden.
  • The desktop nav still looks unchanged.
• Ran pnpm lint.
• Ran pnpm typecheck.