Move session export into the sidebar menu
Add desktop save-and-reveal flow for exported markdown
Show empty-state UI instead of a loading skeleton for empty sessions
Add a download button in the chat view that exports the current session's
messages as a formatted Markdown file. User and assistant turns are
separated by horizontal rules. The filename is derived from the session
title and today's date (e.g. fix-login-bug-2026-04-17.md).