fix: elevate CSS priority to override GTK4 user theme (v0.7.2)

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

- Use STYLE_PROVIDER_PRIORITY_USER for app CSS provider
- Replace border-radius: 50% with 9999px (GTK4 CSS percentage quirk)

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "moonset"
-version = "0.7.1"
+version = "0.7.2"
 edition = "2024"
 description = "Wayland session power menu with GTK4 and Layer Shell"
 license = "MIT"