This PR introduces /btw side-question threads. From the main session, users
can run /btw ... to create a child session (side thread), and a lightweight
in-stream marker card is inserted into the parent chat so you can jump to the
thread quickly. The Sessions list is updated to better represent parent/child
relationships, and related i18n strings are added.

On Desktop, it adds BTW Tauri commands btw_ask, btw_ask_stream, and btw_cance
l, with streaming delivered via btw://text-chunk, btw://completed, and
btw://error events. It also includes a UX fix to make marker focusing stable
after scrolling in the virtualized message list (wait for Virtuoso to settle
and use auto scroll behavior to avoid recycled DOM issues).