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>

app/learn/page.tsx

@@ -2,6 +2,7 @@ import Link from "next/link";
 import PageShell from "@/components/page-shell";
 import { GuaGrid } from "@/components/learn/gua-grid";
 import { ZenCard } from "@/components/ui/zen-card";
+import { APP_CONTAINER } from "@/lib/layout";
 import {
   getGuaName,
   guaNumFromMark,
@@ -17,7 +18,7 @@ export default async function LearnPage() {
   }));
 
   return (
-    <PageShell className="max-w-6xl px-4 py-8">
+    <PageShell className={`${APP_CONTAINER} py-8`}>
       <div className="mb-8 text-center">
         <h1 className="text-2xl font-bold tracking-wide">易经学习</h1>
         <p className="mt-2 text-sm text-muted-foreground">