fix(ui): improvements around folder download (#7036)
Followup to 3c68399eb065bae6100c343025691cae7f15d9f5.
* remove unneeded copy-pasted classes
* fix border radius
* avoid misuse of `.clone-panel` by extending the rule
Preview:

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7036
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>