Fix Chinese filename encoding in ZIP extraction

[Copilot]

ZIP archives created on Windows with Chinese filenames use GBK/GB18030 encoding. Without explicit charset configuration, filenames display as mojibake (e.g., 1-�� - 1����.STL instead of 装配件1 - 盖-2.STL).

Changes

• Configure ZIP decoder with GB18030 encoding: Added newZipFormat() helper that creates archives.Zip with TextEncoding: simplifiedchinese.GB18030
• Apply to all ZIP extraction paths: Updated both openArchive() for regular ZIP files and extractZipMultiPart() for split archives
• Add test coverage: New test creates GBK-encoded ZIP with Chinese filenames and verifies proper extraction

Implementation

func newZipFormat() archives.Zip {
    return archives.Zip{
        TextEncoding: simplifiedchinese.GB18030,
    }
}

// In openArchive()
if _, ok := format.(archives.Zip); ok {
    format = newZipFormat()
}

The mholt/archives library checks the UTF-8 flag in ZIP headers and uses TextEncoding for non-UTF8 filenames. GB18030 is a superset of GBK and handles legacy Chinese-encoded archives.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>解压文件名乱码</issue_title>
<issue_description>中文乱码
</issue_description>

<agent_instructions>Fix it and add test case</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@monkeyWie 自动解压乱码吗？

• Fixes 解压文件名乱码 #1259

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