fix: elevate CSS priority to override GTK4 user theme (v0.6.4)

Colloid-Catppuccin theme loaded via ~/.config/gtk-4.0/gtk.css at
PRIORITY_USER (800) was overriding moonlock'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)

src/main.rs

@@ -24,7 +24,7 @@ fn load_css(display: &gdk::Display) {
     gtk::style_context_add_provider_for_display(
         display,
         &css_provider,
-        gtk::STYLE_PROVIDER_PRIORITY_APPLICATION,
+        gtk::STYLE_PROVIDER_PRIORITY_USER,
     );
 }