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)

resources/style.css

@@ -23,7 +23,7 @@ window.lockscreen.visible {
 
 /* Round avatar image */
 .avatar {
-    border-radius: 50%;
+    border-radius: 9999px;
     min-width: 128px;
     min-height: 128px;
     background-color: @theme_selected_bg_color;