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/GfxRenderer/BitmapHelpers.cpp

@@ -88,3 +88,19 @@ uint8_t quantize(int gray, int x, int y) {
     return quantizeSimple(gray);
   }
 }
+
+// 1-bit noise dithering for fast home screen rendering
+// Uses hash-based noise for consistent dithering that works well at small sizes
+uint8_t quantize1bit(int gray, int x, int y) {
+  gray = adjustPixel(gray);
+
+  // Generate noise threshold using integer hash (no regular pattern to alias)
+  uint32_t hash = static_cast<uint32_t>(x) * 374761393u + static_cast<uint32_t>(y) * 668265263u;
+  hash = (hash ^ (hash >> 13)) * 1274126177u;
+  const int threshold = static_cast<int>(hash >> 24);  // 0-255
+
+  // Simple threshold with noise: gray >= (128 + noise offset) -> white
+  // The noise adds variation around the 128 midpoint
+  const int adjustedThreshold = 128 + ((threshold - 128) / 2);  // Range: 64-192
+  return (gray >= adjustedThreshold) ? 1 : 0;
+}