Render variant and transcript content as markdown on toggle

Adds a hover-revealed eye/code toggle to the variant editor and the
transcript view. In rendered mode the variant textarea is replaced
with a styled markdown view, and partial text/thinking events in the
transcript render as markdown while tool calls and turn markers keep
their structured rendering. Supports CommonMark, GFM (tables, task
lists, strikethrough, autolinks, footnotes), and highlight.js syntax
highlighting themed via the existing light/dark CSS variables.

Author: slaFFik