Fix GBK filename decoding and file handle leaks in archive decompression

[Copilot]

Decompressing ZIP archives with GBK-encoded filenames (common in Chinese Windows systems) fails with invalid argument errors. Large file extraction occasionally fails with unexpected EOF due to file handle conflicts.

Changes

• New ZipFS wrapper (server/filesystem/archiverext/zip.go): Implements fs.FS with automatic GBK→UTF-8 filename transcoding. Detects non-UTF-8 filenames and decodes them as GBK transparently during filesystem operations.

• File handle cleanup: Added io.Closer implementation to ZipFS and proper defer close() in SpaceAvailableForDecompression() to prevent handle leaks between space check and extraction phases.

• Test coverage: Added test case validating SpaceAvailableForDecompression() with GBK-encoded filenames.

Implementation

The ZipFS wrapper intercepts all filename operations and applies this heuristic:

func decodeZipFilename(filename string) string {
    if utf8.ValidString(filename) {
        return filename
    }
    decoded, err := simplifiedchinese.GBK.NewDecoder().String(filename)
    if err != nil {
        return filename
    }
    return decoded
}

This handles the vast majority of Chinese ZIP archives while remaining transparent to UTF-8 archives.

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