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/header.tsx

@@ -1,8 +1,9 @@
 "use client";
 import Link from "next/link";
 import { usePathname } from "next/navigation";
-import { ChatGPT } from "@/components/svg";
+import { TaijiIcon } from "@/components/svg/taiji";
 import { ModeToggle } from "@/components/mode-toggle";
+import { APP_CONTAINER } from "@/lib/layout";
 
 const NAV_ITEMS = [
   { href: "/learn", label: "易经学习" },
@@ -34,11 +35,11 @@ function NavLink({ href, label }: { href: string; label: string }) {
 
 export default function Header() {
   return (
-    <header className="border-b border-border/40 bg-card/60 py-3 shadow-sm backdrop-blur-md">
-      <div className="mx-auto flex w-full max-w-6xl flex-col gap-2 px-4 sm:flex-row sm:items-center sm:justify-between">
+    <header className="relative z-10 border-b border-border/40 bg-card/70 py-3 shadow-sm backdrop-blur-md">
+      <div className={`${APP_CONTAINER} flex flex-col gap-2 sm:flex-row sm:items-center sm:justify-between`}>
         <Link href="/" className="flex items-center justify-center gap-2 sm:justify-start">
-          <ChatGPT />
-          <span className="font-medium">知命阁</span>
+          <TaijiIcon />
+          <span className="font-medium tracking-wide">知命阁</span>
         </Link>
         <nav className="flex flex-wrap items-center justify-center gap-x-4 gap-y-1">
           {NAV_ITEMS.map((item) => (