feat(douyu): 兑换成功后生成结果图片并自动复制到剪贴板

- 账号表格新增兑换图片列，展示每账号最近兑换成功的结果卡片
- 新完成兑换任务自动生成图片并复制到剪贴板（批量时最后完成者覆盖）
- canvas 绘制兑换卡片：商品图/商品名/游戏角色/区服/兑换账号/时间
- 后端兑换成功结果补充 game_name/game_channel/account_name 字段
- 不支持剪贴板的环境降级为点击查看大图

web/frontend/src/components/ExchangeResultImage.tsx

@@ -0,0 +1,52 @@
+import { useEffect, useState } from 'react';
+import { Button, Spin } from 'antd';
+import type { DouyuTaskItem } from '../api/types';
+import { getExchangeImage } from '../utils/exchangeImage';
+
+interface Props {
+  task: DouyuTaskItem;
+  onClick?: (task: DouyuTaskItem, url: string) => void;
+}
+
+export default function ExchangeResultImage({ task, onClick }: Props) {
+  const [url, setUrl] = useState('');
+  const [failed, setFailed] = useState(false);
+  const [gen, setGen] = useState(0);
+
+  useEffect(() => {
+    let alive = true;
+    setFailed(false);
+    getExchangeImage(task)
+      .then(({ url }) => { if (alive) setUrl(url); })
+      .catch(() => { if (alive) setFailed(true); });
+    return () => { alive = false; };
+  }, [task, gen]);
+
+  if (failed) {
+    return (
+      <Button
+        size="small"
+        type="text"
+        onClick={() => setGen((g) => g + 1)}
+      >
+        重试
+      </Button>
+    );
+  }
+  if (!url) return <Spin size="small" />;
+  return (
+    <div
+      onClick={() => onClick?.(task, url)}
+      style={{ cursor: 'pointer', display: 'inline-block', lineHeight: 0 }}
+      title="点击查看大图并复制"
+    >
+      <img
+        src={url}
+        width={56}
+        height={82}
+        alt="兑换结果"
+        style={{ objectFit: 'cover', borderRadius: 6, border: '1px solid #f0f0f0' }}
+      />
+    </div>
+  );
+}