The composer renders Markdown styling (bold, italic, code,
strikethrough) via a Tiptap surface while the stored prompt stays
Markdown. Markers reveal when the caret touches styled text. Chips,
surround-typing, clipboard fragments, citation comments, and cursor
coordinates behave as on the plain surface. Also adds list
continuation/indentation, task-list checkboxes, Mod+B precedence over
the sidebar toggle, and caret-follow scrolling on both surfaces.