fix(ui): replace incorrect class - #1653 - #1654

Issue: #1653

Change: class name replaced in DesignSystemPage.
Impact: No functional changes; styling-only.
Testing: Verified the affected UI.

Before you submit: PRs for new features that haven't been discussed and accepted may be closed. This isn't anti-AI — it's about reviewer burden and long-term maintainability. Please open an issue first. See discussion #1529.

Checklist

• This PR is linked to a GitHub issue (bug fix or an agreed change).
• New feature only: it relates to a suggestion on https://feedback.fider.io and a maintainer has confirmed on the linked issue that it's accepted onto the roadmap.
• I've read the Contributing guide and the feature policy in discussion #1529.