ui: backdrop pan — monotonic top→bottom drift

The S→E→SE→NE loop wandered back near its start, so on a long page the
per-screen movement was imperceptible (worse once the image was zoomed to
215%). Replace it with a single continuous vertical pan (~40% travel) so the
board obviously drifts downward as the page scrolls.

src/styles/global.css

@@ -428,18 +428,15 @@ svg {
     }
   }
 }
+/* Clean monotonic top→bottom pan: as the page scrolls, the board slowly drifts
+   so the motion is always obvious (no wandering loop that nets to zero). ~40% of
+   the 215%-tall image = a long, continuous travel. */
 @keyframes bgPan {
-  0% {
-    transform: translate(-50%, -38%);
+  from {
+    transform: translate(-50%, -28%);
   }
-  35% {
-    transform: translate(-50%, -62%);
-  }
-  68% {
-    transform: translate(-62%, -62%);
-  }
-  100% {
-    transform: translate(-62%, -38%);
+  to {
+    transform: translate(-50%, -72%);
   }
 }