fix(ui): scroll-coupled backdrop pan that works in every browser

CSS animation-timeline: scroll() is Chromium-only, so the backdrop didn't move
for non-Chromium browsers. Replace it with a tiny rAF-throttled scroll handler
in site.js (the existing same-origin script — CSP stays strict, no inline JS)
that drives a --bg-shift CSS var off scroll position. The board now pans
vertically in lock-step with scroll and sits still at rest, in all browsers.

src/styles/global.css

@@ -372,12 +372,15 @@ svg {
   top: 50%;
   left: 50%;
   /* Zoomed in — larger than viewport so the board reads big/immersive and the
-     scroll-pan has room to travel. */
+     scroll-pan has room to travel without ever revealing an edge. */
   width: 215%;
   height: 215%;
   object-fit: cover;
-  transform: translate(-50%, -50%);
+  /* --bg-shift is driven by the scroll handler in site.js (scroll-coupled pan:
+     moves only as the page scrolls). Defaults to 0 → centred when JS is off. */
+  transform: translate(-50%, calc(-50% + var(--bg-shift, 0px)));
   will-change: transform;
+  transition: transform 0.15s linear;
 }
 .site-bg__img--light {
   display: none;
@@ -420,43 +423,6 @@ svg {
       rgba(246, 248, 251, 0.82) 100%
     );
 }
-/* BASELINE motion (works in EVERY browser, no JS): a slow, continuous top↔bottom
-   drift. This is the guaranteed fallback — Firefox/Safari don't support
-   scroll-driven animations, so without this the board would sit dead-still. */
-@media (prefers-reduced-motion: no-preference) {
-  .site-bg__img {
-    animation: bgDrift 48s ease-in-out infinite alternate;
-  }
-}
-/* ENHANCEMENT: where scroll-driven animations exist (Chromium), couple the pan
-   to scroll position instead — overrides the timed drift on the same element. */
-@supports (animation-timeline: scroll()) {
-  @media (prefers-reduced-motion: no-preference) {
-    .site-bg__img {
-      animation: bgPan linear both;
-      animation-timeline: scroll(root block);
-    }
-  }
-}
-/* Continuous timed drift (fallback) — gentle, alternates so it never jumps. */
-@keyframes bgDrift {
-  from {
-    transform: translate(-50%, -38%);
-  }
-  to {
-    transform: translate(-50%, -62%);
-  }
-}
-/* Scroll-coupled monotonic top→bottom pan (Chromium): ~44% travel of the
-   215%-tall image, tied directly to page scroll progress. */
-@keyframes bgPan {
-  from {
-    transform: translate(-50%, -28%);
-  }
-  to {
-    transform: translate(-50%, -72%);
-  }
-}
 
 @media (prefers-reduced-motion: reduce) {
   .scroll-progress {