Resolve helper programs and session dirs without FHS paths

systemctl was invoked as /usr/bin/systemctl, and session directories were
fixed to /usr/share. Both assume an FHS layout and fail on NixOS.

Programs now resolve via PATH; session directories walk XDG_DATA_DIRS with
/usr/share as fallback. Behaviour on Arch is unchanged.

src/power.rs

@@ -105,7 +105,7 @@ fn run_command(action: &'static str, program: &str, args: &[&str]) -> Result<(),
 /// agent — the greeter session has none, so without it a denied authorization
 /// would hang instead of failing fast.
 pub fn reboot() -> Result<(), PowerError> {
-    run_command("reboot", "/usr/bin/systemctl", &["--no-ask-password", "reboot"])
+    run_command("reboot", "systemctl", &["--no-ask-password", "reboot"])
 }
 
 /// Shut down the system via systemctl.
@@ -113,7 +113,7 @@ pub fn reboot() -> Result<(), PowerError> {
 /// `--no-ask-password` for the same reason as [`reboot`] — the agent-less
 /// greeter session has nothing to answer an authorization challenge.
 pub fn shutdown() -> Result<(), PowerError> {
-    run_command("shutdown", "/usr/bin/systemctl", &["--no-ask-password", "poweroff"])
+    run_command("shutdown", "systemctl", &["--no-ask-password", "poweroff"])
 }
 
 #[cfg(test)]

src/sessions.rs

@@ -4,8 +4,34 @@
 use std::fs;
 use std::path::{Path, PathBuf};
 
-const DEFAULT_WAYLAND_DIRS: &[&str] = &["/usr/share/wayland-sessions"];
-const DEFAULT_XSESSION_DIRS: &[&str] = &["/usr/share/xsessions"];
+/// Fallback data directory used when XDG_DATA_DIRS is unset or does not
+/// contain the session directory.
+const FALLBACK_DATA_DIR: &str = "/usr/share";
+
+/// Build the search path for a session directory.
+///
+/// Walks XDG_DATA_DIRS as specified by the XDG Base Directory spec and
+/// appends `/usr/share` as a fallback, so setups that place session files
+/// outside the FHS layout are found as well.
+fn session_search_dirs(subdir: &str) -> Vec<PathBuf> {
+    session_search_dirs_from(&std::env::var("XDG_DATA_DIRS").unwrap_or_default(), subdir)
+}
+
+/// Build the search path from an explicit XDG_DATA_DIRS value.
+fn session_search_dirs_from(data_dirs: &str, subdir: &str) -> Vec<PathBuf> {
+    let mut dirs: Vec<PathBuf> = data_dirs
+        .split(':')
+        .filter(|s| !s.is_empty())
+        .map(|d| PathBuf::from(d).join(subdir))
+        .collect();
+
+    let fallback = PathBuf::from(FALLBACK_DATA_DIR).join(subdir);
+    if !dirs.contains(&fallback) {
+        dirs.push(fallback);
+    }
+
+    dirs
+}
 
 /// Represents an available login session.
 #[derive(Debug, Clone)]
@@ -82,10 +108,8 @@ pub fn get_sessions(
     wayland_dirs: Option<&[PathBuf]>,
     xsession_dirs: Option<&[PathBuf]>,
 ) -> Vec<Session> {
-    let default_wayland: Vec<PathBuf> =
-        DEFAULT_WAYLAND_DIRS.iter().map(PathBuf::from).collect();
-    let default_xsession: Vec<PathBuf> =
-        DEFAULT_XSESSION_DIRS.iter().map(PathBuf::from).collect();
+    let default_wayland = session_search_dirs("wayland-sessions");
+    let default_xsession = session_search_dirs("xsessions");
 
     let wayland = wayland_dirs.unwrap_or(&default_wayland);
     let xsession = xsession_dirs.unwrap_or(&default_xsession);
@@ -129,6 +153,52 @@ mod tests {
         fs::write(dir.join(name), content).unwrap();
     }
 
+    #[test]
+    fn search_dirs_walk_xdg_data_dirs() {
+        let dirs = session_search_dirs_from(
+            "/run/current-system/sw/share:/usr/local/share",
+            "wayland-sessions",
+        );
+        assert_eq!(
+            dirs,
+            vec![
+                PathBuf::from("/run/current-system/sw/share/wayland-sessions"),
+                PathBuf::from("/usr/local/share/wayland-sessions"),
+                PathBuf::from("/usr/share/wayland-sessions"),
+            ]
+        );
+    }
+
+    #[test]
+    fn search_dirs_fall_back_when_unset() {
+        let dirs = session_search_dirs_from("", "xsessions");
+        assert_eq!(dirs, vec![PathBuf::from("/usr/share/xsessions")]);
+    }
+
+    #[test]
+    fn search_dirs_do_not_duplicate_fallback() {
+        let dirs = session_search_dirs_from("/usr/share:/opt/share", "wayland-sessions");
+        assert_eq!(
+            dirs,
+            vec![
+                PathBuf::from("/usr/share/wayland-sessions"),
+                PathBuf::from("/opt/share/wayland-sessions"),
+            ]
+        );
+    }
+
+    #[test]
+    fn search_dirs_skip_empty_segments() {
+        let dirs = session_search_dirs_from("::/opt/share:", "wayland-sessions");
+        assert_eq!(
+            dirs,
+            vec![
+                PathBuf::from("/opt/share/wayland-sessions"),
+                PathBuf::from("/usr/share/wayland-sessions"),
+            ]
+        );
+    }
+
     #[test]
     fn parse_valid_desktop_file() {
         let dir = tempfile::tempdir().unwrap();