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/result-ai.tsx

@@ -2,6 +2,7 @@ import React, { useEffect, useRef, useState } from "react";
 import { RotateCw } from "lucide-react";
 import Markdown from "react-markdown";
 import { Button } from "@/components/ui/button";
+import { TaijiIcon } from "@/components/svg/taiji";
 import { cn } from "@/lib/utils";
 
 function ResultAI({
@@ -85,8 +86,8 @@ function ResultAI({
           <p className="text-sm text-muted-foreground">正在等待 AI 响应...</p>
         ) : (
           <div className="flex h-full min-h-[200px] flex-col items-center justify-center text-center text-muted-foreground">
-            <p className="text-4xl opacity-20">☯</p>
-            <p className="mt-3 max-w-[200px] text-sm leading-relaxed">
+            <TaijiIcon className="h-16 w-16 opacity-25" />
+            <p className="mt-3 max-w-[220px] text-sm leading-relaxed">
               {emptyHint}
             </p>
           </div>