feat(power): logout via loginctl, not Niri-locked

Default `loginctl terminate-session $XDG_SESSION_ID`; `logout_command`
TOML override for other compositors.

src/config.rs

@@ -21,6 +21,9 @@ fn default_config_paths() -> Vec<PathBuf> {
 pub struct Config {
     pub background_path: Option<String>,
     pub background_blur: Option<f32>,
+    /// Override for the logout command (space-separated program + args).
+    /// When unset, logout terminates the logind session via `loginctl`.
+    pub logout_command: Option<String>,
 }
 
 /// Load config from TOML files. Later paths override earlier ones.
@@ -37,6 +40,9 @@ pub fn load_config(config_paths: Option<&[PathBuf]>) -> Config {
                     if parsed.background_path.is_some() {
                         merged.background_path = parsed.background_path;
                     }
+                    if parsed.logout_command.is_some() {
+                        merged.logout_command = parsed.logout_command;
+                    }
                     // Validate blur per source — invalid values preserve the previous default
                     if parsed.background_blur.is_some_and(|b| b.is_finite() && (0.0..=200.0).contains(&b)) {
                         merged.background_blur = parsed.background_blur;
@@ -129,6 +135,17 @@ mod tests {
         let config = Config::default();
         assert!(config.background_path.is_none());
         assert!(config.background_blur.is_none());
+        assert!(config.logout_command.is_none());
+    }
+
+    #[test]
+    fn load_config_reads_logout_command() {
+        let dir = tempfile::tempdir().unwrap();
+        let conf = dir.path().join("moonset.toml");
+        fs::write(&conf, "logout_command = \"niri msg action quit\"\n").unwrap();
+        let paths = vec![conf];
+        let config = load_config(Some(&paths));
+        assert_eq!(config.logout_command.as_deref(), Some("niri msg action quit"));
     }
 
     #[test]

src/power.rs

@@ -125,9 +125,50 @@ pub fn lock() -> Result<(), PowerError> {
     Ok(())
 }
 
-/// Quit the Niri compositor (logout).
+/// Resolve the logout command into program + arguments.
+///
+/// Priority: a user-configured `logout_command` (space-separated) overrides
+/// everything. Otherwise the logind session is terminated via
+/// `loginctl terminate-session <id>` — compositor-agnostic, since every
+/// pam_systemd session sets `XDG_SESSION_ID`.
+fn resolve_logout_command(
+    override_cmd: Option<&str>,
+    session_id: Option<&str>,
+) -> Result<Vec<String>, PowerError> {
+    if let Some(cmd) = override_cmd {
+        let parts: Vec<String> = cmd.split_whitespace().map(str::to_string).collect();
+        if parts.is_empty() {
+            return Err(PowerError::CommandFailed {
+                action: "logout",
+                message: "logout_command is empty".to_string(),
+            });
+        }
+        return Ok(parts);
+    }
+
+    match session_id {
+        Some(id) if !id.is_empty() => Ok(vec![
+            "/usr/bin/loginctl".to_string(),
+            "terminate-session".to_string(),
+            id.to_string(),
+        ]),
+        _ => Err(PowerError::CommandFailed {
+            action: "logout",
+            message: "XDG_SESSION_ID unset; set logout_command in moonset.toml".to_string(),
+        }),
+    }
+}
+
+/// End the session (logout).
+///
+/// Terminates the logind session by default; honours a `logout_command`
+/// override from the config for non-logind or compositor-specific setups.
 pub fn logout() -> Result<(), PowerError> {
-    run_command("logout", "/usr/bin/niri", &["msg", "action", "quit"])
+    let config = crate::config::load_config(None);
+    let session_id = std::env::var("XDG_SESSION_ID").ok();
+    let parts = resolve_logout_command(config.logout_command.as_deref(), session_id.as_deref())?;
+    let args: Vec<&str> = parts[1..].iter().map(String::as_str).collect();
+    run_command("logout", &parts[0], &args)
 }
 
 /// Hibernate the system via systemctl.
@@ -192,4 +233,47 @@ mod tests {
         let result = run_command("test", "echo", &["hello", "world"]);
         assert!(result.is_ok());
     }
+
+    #[test]
+    fn logout_default_uses_loginctl_with_session_id() {
+        let parts = resolve_logout_command(None, Some("3")).unwrap();
+        assert_eq!(parts, vec!["/usr/bin/loginctl", "terminate-session", "3"]);
+    }
+
+    #[test]
+    fn logout_override_takes_precedence() {
+        let parts = resolve_logout_command(Some("niri msg action quit"), Some("3")).unwrap();
+        assert_eq!(parts, vec!["niri", "msg", "action", "quit"]);
+    }
+
+    #[test]
+    fn logout_override_ignores_session_id() {
+        // An override resolves even without a session id.
+        let parts = resolve_logout_command(Some("/usr/bin/swaymsg exit"), None).unwrap();
+        assert_eq!(parts, vec!["/usr/bin/swaymsg", "exit"]);
+    }
+
+    #[test]
+    fn logout_default_errors_without_session_id() {
+        assert!(matches!(
+            resolve_logout_command(None, None),
+            Err(PowerError::CommandFailed { action: "logout", .. })
+        ));
+    }
+
+    #[test]
+    fn logout_default_errors_on_empty_session_id() {
+        assert!(matches!(
+            resolve_logout_command(None, Some("")),
+            Err(PowerError::CommandFailed { action: "logout", .. })
+        ));
+    }
+
+    #[test]
+    fn logout_override_errors_when_blank() {
+        assert!(matches!(
+            resolve_logout_command(Some("   "), Some("3")),
+            Err(PowerError::CommandFailed { action: "logout", .. })
+        ));
+    }
 }