feat(desktop): add a settings entry for unauthenticated users

Show a preferences button next to the login button when logged out, so
guests can open the settings modal without signing in. The modal already
supports guest mode and gates account-bound tabs behind login.

Also fix KbdCombined splitting the comma key in shortcuts like "$mod+,"
as if it were a separator between multiple shortcuts.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>