docs: update README, fix build.rs comment, correct gtk-theme in config

README: replace LD_PRELOAD with MOONGREET_NO_LAYER_SHELL env var,
add missing features (GPU blur, journal logging, password wiping).
build.rs: remove wallpaper.jpg reference.
moongreet.toml: correct gtk-theme to Colloid-Grey-Dark-Catppuccin.

build.rs

@@ -1,5 +1,5 @@
 // ABOUTME: Build script for compiling GResource bundle.
-// ABOUTME: Bundles style.css, wallpaper.jpg, and default-avatar.svg into the binary.
+// ABOUTME: Bundles style.css and default-avatar.svg into the binary.
 
 fn main() {
     glib_build_tools::compile_resources(