fix: elevate CSS priority to override GTK4 user theme (v0.6.1)

Colloid-Catppuccin theme loaded via ~/.config/gtk-4.0/gtk.css at
PRIORITY_USER (800) was overriding moongreet's PRIORITY_APPLICATION (600),
causing avatar to lose its circular border-radius.

- Use STYLE_PROVIDER_PRIORITY_USER for app CSS provider
- Replace border-radius: 50% with 9999px (GTK4 CSS percentage quirk)
- Include missed Cargo.lock and PKGBUILD updates from v0.6.0

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "moongreet"
-version = "0.6.0"
+version = "0.6.1"
 edition = "2024"
 description = "A greetd greeter for Wayland with GTK4 and Layer Shell"
 license = "MIT"