chore: update DS to v0.1.38, enforce 12px font size floor

Update @cameleer/design-system to v0.1.38 (12px minimum font size).
Replace all 10px and 11px font sizes with 12px across 25 CSS modules
and 5 TSX inline styles to match the new DS floor.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/pages/Admin/UsersTab.tsx

@@ -228,7 +228,7 @@ export default function UsersTab({ highlightId, onHighlightConsumed }: { highlig
                   />
                 </div>
                 {duplicateUsername && (
-                  <span style={{ color: 'var(--error)', fontSize: 11 }}>
+                  <span style={{ color: 'var(--error)', fontSize: 12 }}>
                     Username already exists
                   </span>
                 )}