Support ultrawide layout with equal columns and taiji branding.

Use 50/50 mode workspace columns, widen content to 1920px on fish screens, replace header logo with taiji icon, and add full-page taiji backdrop.

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

components/layout/taiji-backdrop.tsx

@@ -0,0 +1,15 @@
+import { TaijiWatermark } from "@/components/svg/taiji";
+
+/** 全页太极背景水印 */
+export default function TaijiBackdrop() {
+  return (
+    <div
+      aria-hidden
+      className="pointer-events-none fixed inset-0 -z-10 overflow-hidden bg-background/80"
+    >
+      <TaijiWatermark className="absolute left-1/2 top-1/2 h-[min(95vmin,1200px)] w-[min(95vmin,1200px)] -translate-x-1/2 -translate-y-1/2 opacity-90" />
+      <TaijiWatermark className="absolute -right-[8%] top-[6%] h-[min(42vmin,520px)] w-[min(42vmin,520px)] opacity-45" />
+      <TaijiWatermark className="absolute -left-[10%] bottom-[4%] h-[min(38vmin,460px)] w-[min(38vmin,460px)] opacity-40" />
+    </div>
+  );
+}