Swaps the rendering layer only. Chat store logic, keyboard handling, and the
/chat-run Socket.IO contract are untouched, so a mixed UI cannot hide a chat
regression behind a library swap.
Arco's dropdowns take their items from a #content slot of a-doption children
rather than a droplist prop, and Arco Triggers cannot be nested, so the two
toolbar menus lost their a-tooltip in favour of a native title. Both rules are
recorded in docs/arco-mapping.md, and tests/e2e/chat-input-dropdown.spec.ts plus
tests/e2e/chat-rendering-smoke.spec.ts keep the menus, the streaming markdown
rendering, and the no-Arco-inside-the-bubble red line from regressing silently.
