SidebarInset (variant=inset) had rounded corners + shadow but no border, so the
content card blended into the bg-sidebar backdrop. Add a subtle border (semantic
--border token: white/6% dark, black/8% light), scoped to the inset variant to
match the existing rounded-xl/shadow.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>