feat: switch logging to systemd journal (v0.4.2)

Replace env_logger + /var/cache/moonlock file logging with
systemd-journal-logger. Logs are now reliably accessible via
journalctl --user -u moonlock, fixing invisible errors in the
systemd user service context.

- Cargo.toml: env_logger → systemd-journal-logger 2.2
- main.rs: setup_logging() uses JournalLog
- PKGBUILD: add systemd-libs dependency
- power.rs: include unstaged systemctl fixes (ABOUTME, --no-ask-password, output())

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "moonlock"
-version = "0.4.1"
+version = "0.4.2"
 edition = "2024"
 description = "A secure Wayland lockscreen with GTK4, PAM and fingerprint support"
 license = "MIT"
@@ -18,7 +18,7 @@ nix = { version = "0.29", features = ["user"] }
 zeroize = { version = "1", features = ["derive"] }
 libc = "0.2"
 log = "0.4"
-env_logger = "0.11"
+systemd-journal-logger = "2.2"
 
 [dev-dependencies]
 tempfile = "3"