webclient: revert LoginScreen to the custom card ...

The kit login-card markup read heavier (uppercase header, wide chrome
border) than the original centered card — restoring the previous
version per user preference.

webclient: LoginScreen rebuilt on kit markup ...

The SSO-only login doesn't fit GnLoginCard's username/password form (the
adapter always renders both fields), so the escape-hatch card is replaced
with the kit's own login-card markup (article.card.login-card + header +
card-content + GnButton) — the same contract kit.css ships for
GnLoginCard, minus the form. Custom card border/title/subtitle styles are
gone; overlay keeps positioning only.

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).