Add cover image display in *Continue Reading* card with framebuffer caching (#200)

## Summary

* **What is the goal of this PR?** (e.g., Fixes a bug in the user
authentication module,

Display the book cover image in the **"Continue Reading"** card on the
home screen, with fast navigation using framebuffer caching.

* **What changes are included?**

- Display book cover image in the "Continue Reading" card on home screen
- Load cover from cached BMP (same as sleep screen cover)
- Add framebuffer store/restore functions (`copyStoredBwBuffer`,
`freeStoredBwBuffer`) for fast navigation after initial render
- Fix `drawBitmap` scaling bug: apply scale to offset only, not to base
coordinates
- Add white text boxes behind title/author/continue reading label for
readability on cover
- Support both EPUB and XTC file cover images
- Increase HomeActivity task stack size from 2048 to 4096 for cover
image rendering

## Additional Context

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

- Performance: First render loads cover from SD card (~800ms),
subsequent navigation uses cached framebuffer (~instant)
- Memory: Framebuffer cache uses ~48KB (6 chunks × 8KB) while on home
screen, freed on exit
- Fallback: If cover image is not available, falls back to standard
text-only display
- The `drawBitmap` fix corrects a bug where screenY = (y + offset) scale
was incorrectly scaling the base coordinates. Now correctly uses screenY
= y + (offset scale)

lib/JpegToBmpConverter/JpegToBmpConverter.h

@@ -5,11 +5,15 @@ class Print;
 class ZipFile;
 
 class JpegToBmpConverter {
-  static void writeBmpHeader(Print& bmpOut, int width, int height);
-  // [COMMENTED OUT] static uint8_t grayscaleTo2Bit(uint8_t grayscale, int x, int y);
   static unsigned char jpegReadCallback(unsigned char* pBuf, unsigned char buf_size,
                                         unsigned char* pBytes_actually_read, void* pCallback_data);
+  static bool jpegFileToBmpStreamInternal(class FsFile& jpegFile, Print& bmpOut, int targetWidth, int targetHeight,
+                                          bool oneBit);
 
  public:
   static bool jpegFileToBmpStream(FsFile& jpegFile, Print& bmpOut);
+  // Convert with custom target size (for thumbnails)
+  static bool jpegFileToBmpStreamWithSize(FsFile& jpegFile, Print& bmpOut, int targetMaxWidth, int targetMaxHeight);
+  // Convert to 1-bit BMP (black and white only, no grays) for fast home screen rendering
+  static bool jpegFileTo1BitBmpStreamWithSize(FsFile& jpegFile, Print& bmpOut, int targetMaxWidth, int targetMaxHeight);
 };