fix(demo): lower IntersectionObserver threshold to prevent feature images from staying hidden

On viewports where height < 15% of the feature grid's total height,
the fade-in animation could never trigger, keeping the entire grid
at opacity 0. Using threshold 0 fires as soon as any part is visible.

Author: alvin000009238

demo/main.js

@@ -227,7 +227,7 @@ function initScrollAnimations() {
         }
       });
     },
-    { threshold: 0.15 }
+    { threshold: 0 }
   );
 
   document.querySelectorAll('.fade-in').forEach((el) => {