feat: switch to systemd-journal-logger, add debug logging (v0.4.0)

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.

CLAUDE.md

@@ -46,7 +46,7 @@ cd pkg && makepkg -sf && sudo pacman -U moongreet-git-<version>-x86_64.pkg.tar.z
 - `i18n.rs` — Locale-Erkennung (LANG / /etc/locale.conf) und String-Tabellen (DE/EN), alle UI- und Login-Fehlermeldungen
 - `config.rs` — TOML-Config ([appearance] background, gtk-theme) + Wallpaper-Fallback
 - `greeter.rs` — GTK4 UI (Overlay-Layout), Login-Flow via greetd IPC, Faillock-Warnung, Avatar-Cache, Last-User/Last-Session Persistence (0o600 Permissions)
-- `main.rs` — Entry Point, GTK App, Layer Shell Setup, Multi-Monitor, Log-Datei (0o640)
+- `main.rs` — Entry Point, GTK App, Layer Shell Setup, Multi-Monitor, systemd-journal-logger
 - `resources/style.css` — Catppuccin-inspiriertes Theme
 
 ## Design Decisions
@@ -59,5 +59,6 @@ cd pkg && makepkg -sf && sudo pacman -U moongreet-git-<version>-x86_64.pkg.tar.z
 - **Symmetrie mit moonset**: Gleiche Patterns (i18n, config, users, power, GResource)
 - **Session-Validierung**: Relative Pfade erlaubt (greetd löst PATH auf), nur `..`/Null-Bytes werden abgelehnt
 - **GTK-Theme-Validierung**: Nur alphanumerisch + `_-+.` erlaubt, verhindert Path-Traversal über Config
-- **File Permissions**: Cache-Dateien 0o600, Log-Datei 0o640
+- **Journal-Logging**: `systemd-journal-logger` statt File-Logging — `journalctl -t moongreet`
+- **File Permissions**: Cache-Dateien 0o600
 - **Testbare Persistence**: `save_*_to`/`load_*_from` Varianten mit konfigurierbarem Pfad für Unit-Tests