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.

src/power.rs

@@ -23,6 +23,7 @@ impl std::error::Error for PowerError {}
 
 /// Run a command and return a PowerError on failure.
 fn run_command(action: &'static str, program: &str, args: &[&str]) -> Result<(), PowerError> {
+    log::debug!("Power action: {action} ({program} {args:?})");
     let child = Command::new(program)
         .args(args)
         .spawn()
@@ -38,6 +39,9 @@ fn run_command(action: &'static str, program: &str, args: &[&str]) -> Result<(),
             message: e.to_string(),
         })?;
 
+    if output.status.success() {
+        log::debug!("Power action {action} completed successfully");
+    }
     if !output.status.success() {
         let stderr = String::from_utf8_lossy(&output.stderr);
         return Err(PowerError::CommandFailed {