Improve initial active connection handling

If the last active connection is not reachable (server down, keychain problems, token revoked, etc.), NeoChat currently fails to load at all, with the only fix being to delete a line from the config file. This is surprisingly hard to fix with a nice UX as long as we stick to the principle of loading the user's last active connection automatically.

This patch thus drops that principle; instead, the user is always asked to choose the connection to continue with.

This intial merge request just focuses on whether we want something like this at all and how it should behave. Follow up merge requests would improve how this relates to other parts of NeoChat (i.e., WelcomePage, account switching, etc.) and the code improvements this enables.