Improve EPUB cover image quality with pre-scaling and Atkinson dithering (#116)

## Summary

* **What is the goal of this PR?**

Replace simple threshold-based grayscale quantization with ordered
dithering using a 4x4 Bayer matrix. This eliminates color banding
artifacts and produces smoother gradients on e-ink display.

* **What changes are included?**

- Add 4x4 Bayer dithering matrix for 16-level threshold patterns
- Modify `grayscaleTo2Bit()` function to accept pixel coordinates and
apply position-based dithering
- Replace simple `grayscale >> 6` threshold with ordered dithering
algorithm that produces smoother gradients

## Additional Context

* Bayer matrix approach: The 4x4 Bayer matrix creates a repeating
pattern that distributes quantization error spatially, effectively
simulating 16 levels of gray using only 4 actual color levels (black,
dark gray, light gray, white).

* Cache invalidation: Existing cached `cover.bmp` files will need to be
deleted to see the improved rendering, as the converter only runs when
the cache is missing.

lib/JpegToBmpConverter/JpegToBmpConverter.h

@@ -6,7 +6,7 @@ class ZipFile;
 
 class JpegToBmpConverter {
   static void writeBmpHeader(Print& bmpOut, int width, int height);
-  static uint8_t grayscaleTo2Bit(uint8_t grayscale);
+  // [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);