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.

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "moonlock"
-version = "0.5.0"
+version = "0.5.1"
 edition = "2024"
 description = "A secure Wayland lockscreen with GTK4, PAM and fingerprint support"
 license = "MIT"
@@ -14,11 +14,10 @@ gdk-pixbuf = "0.22"
 gio = "0.22"
 toml = "0.8"
 serde = { version = "1", features = ["derive"] }
-dirs = "6"
+graphene-rs = { version = "0.22", package = "graphene-rs" }
 nix = { version = "0.29", features = ["user"] }
 zeroize = { version = "1", features = ["derive"] }
 libc = "0.2"
-image = { version = "0.25", default-features = false, features = ["jpeg", "png"] }
 log = "0.4"
 systemd-journal-logger = "2.2"