Login flow improvements:
- Remove "Welcome back!" flash message on successful login
- Auto-login users clicking magic links without confirmation prompt
- Always use "remember me" for magic link logins
- Fix Accounts.login_user_by_magic_link/1 to handle invalid token format
Test fixes:
- Add @tag :integration to Ping tests (require actual system ping)
- Add halt() calls to UserAuth.start_impersonation error paths
Users now go directly to their intended destination after clicking a
magic link, with no interruption for "log in and stay logged in" choice.