fix: audit fixes — double-unlock guard, PAM OOM code, GPU blur, async fp stop (v0.5.1)

Security: prevent double unlock() when PAM and fingerprint succeed
simultaneously (ext-session-lock protocol error). Fix PAM callback
returning PAM_AUTH_ERR instead of PAM_BUF_ERR on calloc OOM.

Performance: replace CPU-side Gaussian blur (image crate) with GPU blur
via GskBlurNode + GskRenderer::render_texture(). Eliminates 500ms-2s
main-thread blocking on cold cache for 4K wallpapers. Remove image and
dirs dependencies (~15 transitive crates). Make fingerprint stop()
fire-and-forget async to avoid 6s UI block after successful auth.

src/fingerprint.rs

@@ -290,7 +290,8 @@ impl FingerprintListener {
     }
 
     /// Stop listening and release the device.
-    /// Uses a short timeout (3s) to avoid blocking the UI indefinitely.
+    /// Signal disconnect is synchronous to prevent further callbacks.
+    /// D-Bus cleanup (VerifyStop + Release) is fire-and-forget to avoid blocking the UI.
     pub fn stop(&mut self) {
         if !self.running {
             return;
@@ -301,20 +302,15 @@ impl FingerprintListener {
             if let Some(id) = self.signal_id.take() {
                 proxy.disconnect(id);
             }
-            let _ = proxy.call_sync(
-                "VerifyStop",
-                None,
-                gio::DBusCallFlags::NONE,
-                3000,
-                gio::Cancellable::NONE,
-            );
-            let _ = proxy.call_sync(
-                "Release",
-                None,
-                gio::DBusCallFlags::NONE,
-                3000,
-                gio::Cancellable::NONE,
-            );
+            let proxy = proxy.clone();
+            glib::spawn_future_local(async move {
+                let _ = proxy
+                    .call_future("VerifyStop", None, gio::DBusCallFlags::NONE, 3000)
+                    .await;
+                let _ = proxy
+                    .call_future("Release", None, gio::DBusCallFlags::NONE, 3000)
+                    .await;
+            });
         }
     }