refactor: 提取 Button 组件，统一按钮变体、尺寸和加载状态

新增 Button.tsx 支持 5 种变体(primary/secondary/danger/ghost/purple)、
3 种尺寸(sm/md/lg)、pill/rounded 形状及内置 loading 状态，
替换 8 个文件中 16 处重复的按钮样板代码。

src/app/error.tsx

@@ -3,6 +3,7 @@
 import { useEffect } from "react";
 import { motion } from "framer-motion";
 import { AlertTriangle, RotateCcw, Home } from "lucide-react";
+import Button from "@/components/Button";
 
 export default function Error({
   error,
@@ -37,20 +38,12 @@ export default function Error({
         </p>
 
         <div className="mt-8 flex gap-3">
-          <button
-            onClick={reset}
-            className="flex items-center gap-1.5 rounded-xl bg-rose-600 px-5 py-2.5 text-sm font-semibold text-white transition-colors hover:bg-rose-500"
-          >
-            <RotateCcw size={15} />
+          <Button onClick={reset} variant="danger" icon={<RotateCcw size={15} />}>
             重试
-          </button>
-          <a
-            href="/"
-            className="flex items-center gap-1.5 rounded-xl bg-surface px-5 py-2.5 text-sm font-semibold text-secondary ring-1 ring-border transition-colors hover:bg-elevated"
-          >
-            <Home size={15} />
+          </Button>
+          <Button onClick={() => window.location.href = "/"} variant="secondary" icon={<Home size={15} />}>
             首页
-          </a>
+          </Button>
         </div>
       </motion.div>
     </div>