Tighten ultrawide layout with visible side gutters.

Cap content at 768-960px via PageShell width tiers, add muted outer margins, and limit gua grid to four columns.

Co-authored-by: Cursor <cursoragent@cursor.com>

components/layout/taiji-backdrop.tsx

@@ -1,15 +1,13 @@
 import { TaijiWatermark } from "@/components/svg/taiji";
 
-/** 全页太极背景水印（居中，不撑满超宽屏） */
+/** 全页太极背景水印 — 仅居中，不铺满超宽屏 */
 export default function TaijiBackdrop() {
   return (
     <div
       aria-hidden
       className="pointer-events-none fixed inset-0 -z-10 overflow-hidden"
     >
-      <TaijiWatermark className="absolute left-1/2 top-1/2 h-[min(72vmin,720px)] w-[min(72vmin,720px)] -translate-x-1/2 -translate-y-1/2" />
-      <TaijiWatermark className="absolute right-[8%] top-[10%] hidden h-[min(28vmin,320px)] w-[min(28vmin,320px)] opacity-60 lg:block" />
-      <TaijiWatermark className="absolute bottom-[8%] left-[8%] hidden h-[min(24vmin,280px)] w-[min(24vmin,280px)] opacity-50 lg:block" />
+      <TaijiWatermark className="absolute left-1/2 top-1/2 h-[min(65vmin,640px)] w-[min(65vmin,640px)] -translate-x-1/2 -translate-y-1/2 opacity-80" />
     </div>
   );
 }