fix(CopyButton): 修复 review 反馈并补充测试 (#66)

* fix: remove non-functional animation classes from CopyButton
* test(CopyButton): 补充单元测试覆盖边界场景
* fix: unify CopyButton imports to named export across all consumers
* fix(CopyButton): add aria-label for screen reader accessibility

src/pages/Base64Converter/TextMode.tsx

@@ -1,7 +1,7 @@
 import { useCallback, useEffect, useMemo, useState } from 'react';
 import TextInputArea from '@/components/TextInputArea';
 import { useI18n } from '@/utils/chromeI18n';
-import CopyButton from '@/components/CopyButton';
+import { CopyButton } from '@/components/CopyButton';
 import { base64ToText, textToBase64 } from '@/utils/base64Converter';
 import SwitchButtonGroup from '@/components/SwitchButtonGroup';
 import { useContextMenuData } from '@/utils/useContextMenuData';