A read-only value cell is now click-to-copy: clicking writes the full,
un-elided value to the system clipboard (copy cursor on hover). One
change in render_ro_row, so it covers every read-only row for every
entity. Adds Message::PropCopyValue routed through iced:📋:write.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>