Translate `library/concurrent.futures.po`

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• Fuzzy: 20
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閱讀 Contributing Guide，然後開啟 library/concurrent.futures.po，處理其中的 fuzzy 和未翻譯項目。檔案達到 100% 翻譯，且 docs.python.org/es/3.14/library/concurrent.futures.html 上呈現的西班牙文頁面不再回退到英文版本，即表示完成。