Display book cover image as background in the Continue Reading card on
the home screen, improving visual identification of the current book.
Key changes:
- Add thumbnail generation (thumb.bmp) for EPUB and XTC/XTCH files
- Uses same dithering/scaling algorithms as sleep screen covers
- Target size: 240x400 (half screen) for optimal Continue Reading card fit
- Add JpegToBmpConverter::jpegFileToBmpStreamWithSize() for custom target sizes
- Add GfxRenderer::copyStoredBwBuffer() and freeStoredBwBuffer() for
framebuffer caching to maintain fast navigation performance
- Add UTF-8 safe string truncation for Korean/CJK text in title/author display
- Draw white boxes behind title/author text for readability over cover image
- Increase HomeActivityTask stack size to 4096 for cover image rendering
- Add bounds checking in XTC thumbnail generation to prevent buffer overflow
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