feat: Add PNG cover image support for EPUB books (#827)

Cherry-pick upstream PR #827 with conflict resolution for mod/master:

- Add PngToBmpConverter library for PNG cover → BMP conversion
- Add PNG thumbnail generation in generateThumbBmp()
- Fix generateCoverBmp() PNG block to use effectiveCoverImageHref
  (consistent with mod's fallback cover candidate probing)
- Add .png to getCoverCandidates() extensions
- Use LOG_ERR macro in ImageToFramebufferDecoder (mod standard)
- Upstream image converter refinements (ImageBlock, PixelCache,
  JpegToFramebufferConverter, PngToFramebufferConverter)

Co-authored-by: Cursor <cursoragent@cursor.com>

lib/PngToBmpConverter/PngToBmpConverter.h

@@ -0,0 +1,14 @@
+#pragma once
+
+class FsFile;
+class Print;
+
+class PngToBmpConverter {
+  static bool pngFileToBmpStreamInternal(FsFile& pngFile, Print& bmpOut, int targetWidth, int targetHeight, bool oneBit,
+                                         bool crop = true);
+
+ public:
+  static bool pngFileToBmpStream(FsFile& pngFile, Print& bmpOut, bool crop = true);
+  static bool pngFileToBmpStreamWithSize(FsFile& pngFile, Print& bmpOut, int targetMaxWidth, int targetMaxHeight);
+  static bool pngFileTo1BitBmpStreamWithSize(FsFile& pngFile, Print& bmpOut, int targetMaxWidth, int targetMaxHeight);
+};