feat: switch to systemd-journal-logger, add debug logging (v0.6.0)

Replace env_logger with systemd-journal-logger for consistent logging
across moonset/moonlock/moongreet. Add MOONSET_DEBUG env var and debug
statements across all modules. Also includes shared blur cache for
multi-monitor and detached moonlock spawn for lock action.

CLAUDE.md

@@ -35,7 +35,7 @@ LD_PRELOAD=/usr/lib/libgtk4-layer-shell.so ./target/release/moonset
 
 ## Architektur
 
-- `main.rs` — Entry Point, GTK App, Layer Shell Setup, Multi-Monitor, zentrale `GRESOURCE_PREFIX`-Konstante
+- `main.rs` — Entry Point, GTK App, Layer Shell Setup, Multi-Monitor, systemd-Journal-Logging, Debug-Level per `MOONSET_DEBUG` Env-Var, zentrale `GRESOURCE_PREFIX`-Konstante
 - `power.rs` — 5 Power-Action-Wrapper mit absoluten Pfaden und 30s Timeout (lock, logout, hibernate, reboot, shutdown)
 - `i18n.rs` — Locale-Erkennung und String-Tabellen (DE/EN)
 - `config.rs` — TOML-Config + Wallpaper-Fallback
@@ -56,3 +56,4 @@ Kurzfassung der wichtigsten Entscheidungen:
 - **Absolute Pfade für Binaries**: `/usr/bin/systemctl` etc. statt relativer Pfade (Security)
 - **GResource-Bundle**: CSS, Wallpaper (komprimiert) und Default-Avatar sind in die Binary kompiliert
 - **Async Power Actions**: `glib::spawn_future_local` + `gio::spawn_blocking` mit 30s Timeout
+- **Journal-Logging**: `systemd-journal-logger` statt File-Logging — `journalctl -t moonset`, Debug-Level per `MOONSET_DEBUG` Env-Var