fix: use systemctl instead of loginctl for reboot/poweroff

loginctl has no reboot/poweroff subcommands — these are systemctl
commands. The error was silently swallowed because stderr wasn't
captured and logs went to a non-existent directory.

src/power.rs

@@ -37,8 +37,8 @@ fn run_command(action: &'static str, program: &str, args: &[&str]) -> Result<(),
     Ok(())
 }
 
-pub fn reboot() -> Result<(), PowerError> { run_command("reboot", "/usr/bin/loginctl", &["reboot"]) }
-pub fn shutdown() -> Result<(), PowerError> { run_command("shutdown", "/usr/bin/loginctl", &["poweroff"]) }
+pub fn reboot() -> Result<(), PowerError> { run_command("reboot", "/usr/bin/systemctl", &["reboot"]) }
+pub fn shutdown() -> Result<(), PowerError> { run_command("shutdown", "/usr/bin/systemctl", &["poweroff"]) }
 
 #[cfg(test)]
 mod tests {