feat: restore book cover/thumbnail prerender on first open

- Add isValidThumbnailBmp(), generateInvalidFormatCoverBmp(), and
  generateInvalidFormatThumbBmp() methods to Epub class for validating
  BMP files and generating X-pattern marker images when cover extraction
  fails (e.g., progressive JPG).
- Restore prerender block in EpubReaderActivity::onEnter() that checks
  for missing cover BMPs (fit + cropped) and thumbnail BMPs at each
  PRERENDER_THUMB_HEIGHTS size, showing a "Preparing book..." popup
  with progress. Falls back to PlaceholderCoverGenerator, then to
  invalid-format marker BMPs as last resort.

Made-with: Cursor

lib/Epub/Epub.h

@@ -56,6 +56,9 @@ class Epub {
   std::string getThumbBmpPath() const;
   std::string getThumbBmpPath(int height) const;
   bool generateThumbBmp(int height) const;
+  bool generateInvalidFormatCoverBmp(bool cropped = false) const;
+  bool generateInvalidFormatThumbBmp(int height) const;
+  static bool isValidThumbnailBmp(const std::string& bmpPath);
   uint8_t* readItemContentsToBytes(const std::string& itemHref, size_t* size = nullptr,
                                    bool trailingNullByte = false) const;
   bool readItemContentsToStream(const std::string& itemHref, Print& out, size_t chunkSize) const;