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.h

@@ -5,8 +5,89 @@
 // Helper functions
 uint8_t quantize(int gray, int x, int y);
 uint8_t quantizeSimple(int gray);
+uint8_t quantize1bit(int gray, int x, int y);
 int adjustPixel(int gray);
 
+// 1-bit Atkinson dithering - better quality than noise dithering for thumbnails
+// Error distribution pattern (same as 2-bit but quantizes to 2 levels):
+//     X  1/8 1/8
+// 1/8 1/8 1/8
+//     1/8
+class Atkinson1BitDitherer {
+ public:
+  explicit Atkinson1BitDitherer(int width) : width(width) {
+    errorRow0 = new int16_t[width + 4]();  // Current row
+    errorRow1 = new int16_t[width + 4]();  // Next row
+    errorRow2 = new int16_t[width + 4]();  // Row after next
+  }
+
+  ~Atkinson1BitDitherer() {
+    delete[] errorRow0;
+    delete[] errorRow1;
+    delete[] errorRow2;
+  }
+
+  // EXPLICITLY DELETE THE COPY CONSTRUCTOR
+  Atkinson1BitDitherer(const Atkinson1BitDitherer& other) = delete;
+
+  // EXPLICITLY DELETE THE COPY ASSIGNMENT OPERATOR
+  Atkinson1BitDitherer& operator=(const Atkinson1BitDitherer& other) = delete;
+
+  uint8_t processPixel(int gray, int x) {
+    // Apply brightness/contrast/gamma adjustments
+    gray = adjustPixel(gray);
+
+    // Add accumulated error
+    int adjusted = gray + errorRow0[x + 2];
+    if (adjusted < 0) adjusted = 0;
+    if (adjusted > 255) adjusted = 255;
+
+    // Quantize to 2 levels (1-bit): 0 = black, 1 = white
+    uint8_t quantized;
+    int quantizedValue;
+    if (adjusted < 128) {
+      quantized = 0;
+      quantizedValue = 0;
+    } else {
+      quantized = 1;
+      quantizedValue = 255;
+    }
+
+    // Calculate error (only distribute 6/8 = 75%)
+    int error = (adjusted - quantizedValue) >> 3;  // error/8
+
+    // Distribute 1/8 to each of 6 neighbors
+    errorRow0[x + 3] += error;  // Right
+    errorRow0[x + 4] += error;  // Right+1
+    errorRow1[x + 1] += error;  // Bottom-left
+    errorRow1[x + 2] += error;  // Bottom
+    errorRow1[x + 3] += error;  // Bottom-right
+    errorRow2[x + 2] += error;  // Two rows down
+
+    return quantized;
+  }
+
+  void nextRow() {
+    int16_t* temp = errorRow0;
+    errorRow0 = errorRow1;
+    errorRow1 = errorRow2;
+    errorRow2 = temp;
+    memset(errorRow2, 0, (width + 4) * sizeof(int16_t));
+  }
+
+  void reset() {
+    memset(errorRow0, 0, (width + 4) * sizeof(int16_t));
+    memset(errorRow1, 0, (width + 4) * sizeof(int16_t));
+    memset(errorRow2, 0, (width + 4) * sizeof(int16_t));
+  }
+
+ private:
+  int width;
+  int16_t* errorRow0;
+  int16_t* errorRow1;
+  int16_t* errorRow2;
+};
+
 // Atkinson dithering - distributes only 6/8 (75%) of error for cleaner results
 // Error distribution pattern:
 //     X  1/8 1/8