Workbench panels used a translucent 1px border on a small radius, which nearly
vanished against the same-coloured shell in dark themes and read as stray pixels
on the anti-aliased arc (most visible on Linux). Introduce .dbx-panel with the
radius one step above rounded-md (so the Corner style setting still applies) and
an opaque border in dark themes, and let the sidebar's inner overflow container
inherit the radius so its header no longer paints over the rounded corner.
Co-authored-by: Claude Sonnet 5.5 <noreply@anthropic.com>
