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.lock

@@ -616,7 +616,7 @@ dependencies = [
 
 [[package]]
 name = "moonset"
-version = "0.7.1"
+version = "0.7.2"
 dependencies = [
  "dirs",
  "gdk-pixbuf",