Keep the peer picker's checkboxes in the primary colour

The round checkbox the media search lays over a thumb - white border,
shadow, green once checked - became a mixin when chats got a selection
mode, and the chat list included it on every `chatlist-chat`. The peer
picker's rows are `chatlist-chat` too, so New Group's member picker,
Forward and every other picker turned green with a shadow.

The chat list's own checkbox no longer lies over the avatar - it has a
lane at the start of the row, as in the picker - so it takes the
picker's look as well: only `.dialog-select-checkbox` is styled, primary
fill over the ring. The search keeps its overlay checkbox, back in
`_searchSuper.scss` where it lived before, and the mixin is gone with
its last user.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>