feat: separate logind feature from systemd

Move logind-zbus to a dedicated 'logind' feature that is independent
of the 'systemd' feature. This allows non-systemd users (e.g., OpenRC
with elogind) to access lid switch inhibition and lid status detection
without requiring the full systemd stack.

The 'systemd' feature now depends on 'logind' to maintain backward
compatibility, so existing users are unaffected.

Feature configuration:
- default: ["systemd"]
- logind: ["logind-zbus"]
- systemd: ["libsystemd", "logind", "tracing-journald"]

Resolves #2473

Coding-Agent: OpenCode
Model: claude-sonnet-4-5

src/dbus/mod.rs

@@ -14,7 +14,7 @@ use tracing::{error, warn};
 
 pub mod a11y_keyboard_monitor;
 use a11y_keyboard_monitor::A11yKeyboardMonitorState;
-#[cfg(feature = "systemd")]
+#[cfg(feature = "logind")]
 pub mod logind;
 mod name_owners;
 mod power;

src/input/mod.rs

@@ -1600,7 +1600,7 @@ impl State {
             }
             InputEvent::Special(_) => {}
             InputEvent::SwitchToggle { event } => {
-                #[cfg(feature = "systemd")]
+                #[cfg(feature = "logind")]
                 if event.switch() == Some(Switch::Lid) && self.common.inhibit_lid_fd.is_some() {
                     let backend = self.backend.lock();
                     let output = backend

src/state.rs

@@ -116,7 +116,7 @@ use smithay::{
 };
 use tracing::warn;
 
-#[cfg(feature = "systemd")]
+#[cfg(feature = "logind")]
 use std::os::fd::OwnedFd;
 
 use std::{
@@ -292,7 +292,7 @@ pub struct Common {
     pub xwayland_shell_state: XWaylandShellState,
     pub pointer_focus_state: Option<PointerFocusState>,
 
-    #[cfg(feature = "systemd")]
+    #[cfg(feature = "logind")]
     pub inhibit_lid_fd: Option<OwnedFd>,
 
     pub with_xwayland: bool,
@@ -791,7 +791,7 @@ impl State {
                 pointer_focus_state: None,
                 dbus_state,
 
-                #[cfg(feature = "systemd")]
+                #[cfg(feature = "logind")]
                 inhibit_lid_fd: None,
 
                 with_xwayland,
@@ -816,7 +816,7 @@ impl State {
     }
 
     fn update_inhibitor_locks(&mut self) {
-        #[cfg(feature = "systemd")]
+        #[cfg(feature = "logind")]
         {
             use smithay::backend::session::Session;
             use tracing::{debug, error, warn};