feat(widgets): AKSRT image slider widget

Lightweight multi-slide carousel with captions, dots, arrows, and CSS
bundled via widgets.css import; registered in widgets-register.

assets/css/frontend/widgets.css

@@ -2,3 +2,4 @@
 @import url("widgets/part-2.css");
 @import url("widgets/part-3.css");
 @import url("widgets/part-4.css");
+@import url("widget-slider.css");