Narrow desktop layout margins and fix taiji icon in light mode.

Cap content width at 1152px with side margins on ultrawide screens, add stroke to taiji logo for light backgrounds, and shrink backdrop watermark.

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

app/page.tsx

@@ -2,7 +2,8 @@ import Link from "next/link";
 import PageShell from "@/components/page-shell";
 import { BookOpen, BrainCircuit, Compass, Sparkles } from "lucide-react";
 import { ZenCard } from "@/components/ui/zen-card";
-import { APP_CONTAINER } from "@/lib/layout";
+import { HOME_CONTAINER } from "@/lib/layout";
+import { TaijiIcon } from "@/components/svg/taiji";
 
 const MODULES = [
   {
@@ -37,9 +38,9 @@ const MODULES = [
 
 export default function Home() {
   return (
-    <PageShell className={`${APP_CONTAINER} py-10 sm:py-14`}>
+    <PageShell className={`${HOME_CONTAINER} py-10 sm:py-14`}>
       <div className="mb-10 text-center">
-        <p className="text-4xl opacity-30">☯</p>
+        <TaijiIcon className="mx-auto h-12 w-12 opacity-80" />
         <h1 className="mt-2 text-3xl font-bold tracking-[0.2em]">知命阁</h1>
         <p className="mt-3 text-sm tracking-wide text-muted-foreground">
           融合周易智慧与人工智能