Download and open session files

Goal

Let users download a session as a JSON file and open a session file someone sends them.

Why: before the cloud deploy, this is the only way to back up or share work.
Constraints: the file holds the session's view and track collection JSON; actions live in the session row menu, plus an "Open session file" action next to New session.
Blockers: Add saved session navigation and deletion #336 merging.

Early draft mockup from Sketch, posted as a discussion starter for Friday's feature review. Nothing here is final.

Card layout, four images: the menu, the open dialog, the confirm step, and the error state.

Open question: when a file's session already exists, opening it makes a new copy (for example hbb-lcr-2) instead of overwriting. Is that right?