fix: use same file name as KOReader for OPDS downloads (#1286)

This allows KOSync to work for files downloaded from OPDS when set to
use filename matching

## Summary

* **What is the goal of this PR?** Brings the names of files downloaded
via OPDS into line with the KOReader OPDS plugin so the files sync when
KOSync is set to filename matching
* **What changes are included?** Change the filename generated by the
OPDS browser from "\<title\> - \<author\>" to "\<author\> - \<title\>"
so it matches KOReader.

## Additional Context

* Add any other information that might be helpful for the reviewer
(e.g., performance implications, potential risks,
  specific areas to focus on).

Both KOReader and crosspoint sanitise the filename by removing special
characters. This step seems to already use the same rules.

---

### AI Usage

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Did you use AI tools to help write this code? NO

src/activities/browser/OpdsBookBrowserActivity.cpp

@@ -264,7 +264,7 @@ void OpdsBookBrowserActivity::downloadBook(const OpdsEntry& book) {
   const std::string feedUrl = UrlUtils::buildUrl(server.url, currentPath);
   std::string downloadUrl = UrlUtils::buildUrl(feedUrl, book.href);
   std::string filename =
-      "/" + StringUtils::sanitizeFilename(book.title + (book.author.empty() ? "" : " - " + book.author)) + ".epub";
+      "/" + StringUtils::sanitizeFilename((book.author.empty() ? "" : book.author + " - ") + book.title) + ".epub";
   LOG_DBG("OPDS", "Downloading: %s -> %s", downloadUrl.c_str(), filename.c_str());
 
   const auto result = HttpDownloader::downloadToFile(