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)

resources/style.css

@@ -27,7 +27,7 @@ window.wallpaper.visible {
 
 /* Round avatar image */
 .avatar {
-    border-radius: 50%;
+    border-radius: 9999px;
     min-width: 128px;
     min-height: 128px;
     background-color: @theme_selected_bg_color;
@@ -48,7 +48,7 @@ window.wallpaper.visible {
     min-width: 120px;
     min-height: 120px;
     padding: 16px;
-    border-radius: 50%;
+    border-radius: 9999px;
     background-color: alpha(@theme_base_color, 0.55);
     color: @theme_fg_color;
     border: none;