Replace env_logger file-based logging with systemd-journal-logger for
consistency with moonlock and native journalctl integration. Add debug-level
logging at all decision points: config loading, user/session detection,
avatar resolution, locale detection, IPC messages, login flow, and
persistence. No credentials are ever logged.
Complete rewrite of the greetd greeter from Python/PyGObject to Rust/gtk4-rs
for consistency with moonset, single binary without Python runtime, and
improved security through Rust memory safety.
Modules: main, greeter, ipc, config, i18n, users, sessions, power
86 unit tests covering all modules including login_worker IPC flow.
Security hardening: shell-word splitting for exec_cmd, absolute path
validation for session binaries, session-name sanitization, absolute
loginctl path, atomic IPC writes.
