v2.6.0: multi-account support

Add the ability to store multiple sr.ht personal access tokens and switch
between them without signing out. A new account switcher button (person.crop.circle)
in the top-right of the More tab opens a sheet listing all saved accounts. Tap
an account to switch to it; the app reloads fully on switch. Accounts can be added
or removed from the same sheet. Existing single-token installs are migrated
automatically on first launch.