fix: use HTTPClient::writeToStream for downloading files from OPDS (#1207)

## Summary

* Refactored `HttpDownloader::downloadToFile` to use `FileWriteStream`
and `HTTPClient::writeToStream`, removing manual chunked downloading
logic, which was error-prone.
* Fixes
https://github.com/crosspoint-reader/crosspoint-reader/issues/632

## Additional Context

* Tested downloading files from OPDS with a size up to 10 mb.

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src/network/HttpDownloader.h

@@ -38,7 +38,4 @@ class HttpDownloader {
    */
   static DownloadError downloadToFile(const std::string& url, const std::string& destPath,
                                       ProgressCallback progress = nullptr);
-
- private:
-  static constexpr size_t DOWNLOAD_CHUNK_SIZE = 1024;
 };