fix: elevate CSS priority to override GTK4 user theme (v0.6.1)

Colloid-Catppuccin theme loaded via ~/.config/gtk-4.0/gtk.css at
PRIORITY_USER (800) was overriding moongreet'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)
- Include missed Cargo.lock and PKGBUILD updates from v0.6.0

resources/style.css

@@ -22,7 +22,7 @@ window.wallpaper {
 
 /* Round avatar image — size is set via set_size_request() in code */
 .avatar {
-    border-radius: 50%;
+    border-radius: 9999px;
     min-width: 128px;
     min-height: 128px;
     background-color: @theme_selected_bg_color;