feat(home): Add cover image thumbnail to Continue Reading card

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

lib/Epub/Epub.h

@@ -43,6 +43,8 @@ class Epub {
   const std::string& getAuthor() const;
   std::string getCoverBmpPath() const;
   bool generateCoverBmp() const;
+  std::string getThumbBmpPath() const;
+  bool generateThumbBmp() const;
   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;