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/GfxRenderer/GfxRenderer.cpp

@@ -132,7 +132,9 @@ void GfxRenderer::drawBitmap(const Bitmap& bitmap, const int x, const int y, con
     isScaled = true;
   }
 
-  const uint8_t outputRowSize = (bitmap.getWidth() + 3) / 4;
+  // Calculate output row size (2 bits per pixel, packed into bytes)
+  // IMPORTANT: Use int, not uint8_t, to avoid overflow for images > 1020 pixels wide
+  const int outputRowSize = (bitmap.getWidth() + 3) / 4;
   auto* outputRow = static_cast<uint8_t*>(malloc(outputRowSize));
   auto* rowBytes = static_cast<uint8_t*>(malloc(bitmap.getRowBytes()));
 
@@ -154,7 +156,7 @@ void GfxRenderer::drawBitmap(const Bitmap& bitmap, const int x, const int y, con
       break;
     }
 
-    if (bitmap.readRow(outputRow, rowBytes) != BmpReaderError::Ok) {
+    if (bitmap.readRow(outputRow, rowBytes, bmpY) != BmpReaderError::Ok) {
       Serial.printf("[%lu] [GFX] Failed to read row %d from bitmap\n", millis(), bmpY);
       free(outputRow);
       free(rowBytes);