webclient: apply kit radii to content-cards and viewed-image thumbs, fix dead scss tokens ...

The content-card and ArtifactsPanel style blocks used @use 'kit-deps'
inside <style scoped> without lang=scss, so sass never ran: the
-radius-md token stayed a literal in the built CSS and the
browser saw border-radius: -radius-md (invalid → 0 corners).
Both blocks now declare lang=scss and compile to the kit's 6px radius.

Also rounds the corners of user-message image attachments and adds the
missing label prop on the tools-collapse icon button (kit dev warning).