feat(strip): smooth scroll animation on arrow button clicks

scroll-behavior: smooth on the strip element ensures programmatic
scrollBy calls animate consistently, cooperating with scroll-snap.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Vgmzx8VTTTmCskSpQtsLTr

themes/intotheeast/css/style.css

@@ -217,6 +217,7 @@ body::after {
     display: flex;
     overflow-x: scroll;
     scroll-snap-type: x mandatory;
+    scroll-behavior: smooth;
     scrollbar-width: none;
     height: 100%;
 }