"Restore from backup" button doesn't prompt for SSSS, which is surprising

Clicking on "restore from backup" these days, if you have your backup key cached locally, goes and pulls down all the keys from the server without prompting further.

This is a bit surprising - when is a user meant to actually use this button? Should we hide it if the backup key is already cached?

Start by locating the "Restore from backup" button and the SSSS restore flow in the TypeScript client. Reproduce the behavior with a locally cached backup key, then inspect how the flow decides whether to prompt or retrieve keys. Done should match an explicitly agreed behavior for cached keys, with coverage for both cached and uncached cases.

Written by the indexing model from the issue text.