fix: harden avatar load against symlink TOCTOU (v0.6.19)

SEC-01 (security audit, LOW): the avatar load followed symlinks via
gio::File while the wallpaper load was already O_NOFOLLOW-hardened — the
project's lock-path hardening was applied inconsistently. Share one
read_file_nofollow loader for both file reads so they cannot diverge
again; a symlinked ~/.face now fails open with ELOOP and falls back to
the default avatar. Adds loader unit tests (regular file, symlink->ELOOP).

Bundles clippy cleanup: c"" literal in auth.rs, let-chains, drop
redundant gtk4_session_lock import, blur guard via .filter() (unifies
with moongreet/moonset).

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "moonlock"
-version = "0.6.18"
+version = "0.6.19"
 edition = "2024"
 description = "A secure Wayland lockscreen with GTK4, PAM and fingerprint support"
 license = "MIT"