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.

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "moongreet"
-version = "0.3.2"
+version = "0.4.0"
 edition = "2024"
 description = "A greetd greeter for Wayland with GTK4 and Layer Shell"
 license = "MIT"
@@ -16,7 +16,7 @@ toml = "0.8"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 log = "0.4"
-env_logger = "0.11"
+systemd-journal-logger = "2.2"
 
 [dev-dependencies]
 tempfile = "3"