*: Fix multi-account login flow, avatar size and shape
- Pass shared BlueskyConn to LoginView so new login writes to same
instance, then reset clients via onNewLogin() to force re-creation
- Clear nav back stack and reset firstLoadDone on login so data reloads
- Reduce top bar avatar from 55dp to 40dp
- Top bar avatar respects avatar shape setting
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>