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/Bitmap.cpp

@@ -228,7 +228,10 @@ BmpReaderError Bitmap::readNextRow(uint8_t* data, uint8_t* rowBuffer) const {
     }
     case 1: {
       for (int x = 0; x < width; x++) {
-        lum = (rowBuffer[x >> 3] & (0x80 >> (x & 7))) ? 0xFF : 0x00;
+        // Get palette index (0 or 1) from bit at position x
+        const uint8_t palIndex = (rowBuffer[x >> 3] & (0x80 >> (x & 7))) ? 1 : 0;
+        // Use palette lookup for proper black/white mapping
+        lum = paletteLum[palIndex];
         packPixel(lum);
       }
       break;

lib/GfxRenderer/Bitmap.h

@@ -42,6 +42,8 @@ class Bitmap {
   bool isTopDown() const { return topDown; }
   bool hasGreyscale() const { return bpp > 1; }
   int getRowBytes() const { return rowBytes; }
+  bool is1Bit() const { return bpp == 1; }
+  uint16_t getBpp() const { return bpp; }
 
  private:
   static uint16_t readLE16(FsFile& f);

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;
+}

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

lib/GfxRenderer/GfxRenderer.cpp

@@ -154,6 +154,12 @@ void GfxRenderer::drawImage(const uint8_t bitmap[], const int x, const int y, co
 
 void GfxRenderer::drawBitmap(const Bitmap& bitmap, const int x, const int y, const int maxWidth, const int maxHeight,
                              const float cropX, const float cropY) const {
+  // For 1-bit bitmaps, use optimized 1-bit rendering path (no crop support for 1-bit)
+  if (bitmap.is1Bit() && cropX == 0.0f && cropY == 0.0f) {
+    drawBitmap1Bit(bitmap, x, y, maxWidth, maxHeight);
+    return;
+  }
+
   float scale = 1.0f;
   bool isScaled = false;
   int cropPixX = std::floor(bitmap.getWidth() * cropX / 2.0f);
@@ -195,6 +201,9 @@ void GfxRenderer::drawBitmap(const Bitmap& bitmap, const int x, const int y, con
     if (screenY >= getScreenHeight()) {
       break;
     }
+    if (screenY < 0) {
+      continue;
+    }
 
     if (bitmap.readNextRow(outputRow, rowBytes) != BmpReaderError::Ok) {
       Serial.printf("[%lu] [GFX] Failed to read row %d from bitmap\n", millis(), bmpY);
@@ -217,6 +226,9 @@ void GfxRenderer::drawBitmap(const Bitmap& bitmap, const int x, const int y, con
       if (screenX >= getScreenWidth()) {
         break;
       }
+      if (screenX < 0) {
+        continue;
+      }
 
       const uint8_t val = outputRow[bmpX / 4] >> (6 - ((bmpX * 2) % 8)) & 0x3;
 
@@ -234,6 +246,143 @@ void GfxRenderer::drawBitmap(const Bitmap& bitmap, const int x, const int y, con
   free(rowBytes);
 }
 
+void GfxRenderer::drawBitmap1Bit(const Bitmap& bitmap, const int x, const int y, const int maxWidth,
+                                 const int maxHeight) const {
+  float scale = 1.0f;
+  bool isScaled = false;
+  if (maxWidth > 0 && bitmap.getWidth() > maxWidth) {
+    scale = static_cast<float>(maxWidth) / static_cast<float>(bitmap.getWidth());
+    isScaled = true;
+  }
+  if (maxHeight > 0 && bitmap.getHeight() > maxHeight) {
+    scale = std::min(scale, static_cast<float>(maxHeight) / static_cast<float>(bitmap.getHeight()));
+    isScaled = true;
+  }
+
+  // For 1-bit BMP, output is still 2-bit packed (for consistency with readNextRow)
+  const int outputRowSize = (bitmap.getWidth() + 3) / 4;
+  auto* outputRow = static_cast<uint8_t*>(malloc(outputRowSize));
+  auto* rowBytes = static_cast<uint8_t*>(malloc(bitmap.getRowBytes()));
+
+  if (!outputRow || !rowBytes) {
+    Serial.printf("[%lu] [GFX] !! Failed to allocate 1-bit BMP row buffers\n", millis());
+    free(outputRow);
+    free(rowBytes);
+    return;
+  }
+
+  for (int bmpY = 0; bmpY < bitmap.getHeight(); bmpY++) {
+    // Read rows sequentially using readNextRow
+    if (bitmap.readNextRow(outputRow, rowBytes) != BmpReaderError::Ok) {
+      Serial.printf("[%lu] [GFX] Failed to read row %d from 1-bit bitmap\n", millis(), bmpY);
+      free(outputRow);
+      free(rowBytes);
+      return;
+    }
+
+    // Calculate screen Y based on whether BMP is top-down or bottom-up
+    const int bmpYOffset = bitmap.isTopDown() ? bmpY : bitmap.getHeight() - 1 - bmpY;
+    int screenY = y + (isScaled ? static_cast<int>(std::floor(bmpYOffset * scale)) : bmpYOffset);
+    if (screenY >= getScreenHeight()) {
+      continue;  // Continue reading to keep row counter in sync
+    }
+    if (screenY < 0) {
+      continue;
+    }
+
+    for (int bmpX = 0; bmpX < bitmap.getWidth(); bmpX++) {
+      int screenX = x + (isScaled ? static_cast<int>(std::floor(bmpX * scale)) : bmpX);
+      if (screenX >= getScreenWidth()) {
+        break;
+      }
+      if (screenX < 0) {
+        continue;
+      }
+
+      // Get 2-bit value (result of readNextRow quantization)
+      const uint8_t val = outputRow[bmpX / 4] >> (6 - ((bmpX * 2) % 8)) & 0x3;
+
+      // For 1-bit source: 0 or 1 -> map to black (0,1,2) or white (3)
+      // val < 3 means black pixel (draw it)
+      if (val < 3) {
+        drawPixel(screenX, screenY, true);
+      }
+      // White pixels (val == 3) are not drawn (leave background)
+    }
+  }
+
+  free(outputRow);
+  free(rowBytes);
+}
+
+void GfxRenderer::fillPolygon(const int* xPoints, const int* yPoints, int numPoints, bool state) const {
+  if (numPoints < 3) return;
+
+  // Find bounding box
+  int minY = yPoints[0], maxY = yPoints[0];
+  for (int i = 1; i < numPoints; i++) {
+    if (yPoints[i] < minY) minY = yPoints[i];
+    if (yPoints[i] > maxY) maxY = yPoints[i];
+  }
+
+  // Clip to screen
+  if (minY < 0) minY = 0;
+  if (maxY >= getScreenHeight()) maxY = getScreenHeight() - 1;
+
+  // Allocate node buffer for scanline algorithm
+  auto* nodeX = static_cast<int*>(malloc(numPoints * sizeof(int)));
+  if (!nodeX) {
+    Serial.printf("[%lu] [GFX] !! Failed to allocate polygon node buffer\n", millis());
+    return;
+  }
+
+  // Scanline fill algorithm
+  for (int scanY = minY; scanY <= maxY; scanY++) {
+    int nodes = 0;
+
+    // Find all intersection points with edges
+    int j = numPoints - 1;
+    for (int i = 0; i < numPoints; i++) {
+      if ((yPoints[i] < scanY && yPoints[j] >= scanY) || (yPoints[j] < scanY && yPoints[i] >= scanY)) {
+        // Calculate X intersection using fixed-point to avoid float
+        int dy = yPoints[j] - yPoints[i];
+        if (dy != 0) {
+          nodeX[nodes++] = xPoints[i] + (scanY - yPoints[i]) * (xPoints[j] - xPoints[i]) / dy;
+        }
+      }
+      j = i;
+    }
+
+    // Sort nodes by X (simple bubble sort, numPoints is small)
+    for (int i = 0; i < nodes - 1; i++) {
+      for (int k = i + 1; k < nodes; k++) {
+        if (nodeX[i] > nodeX[k]) {
+          int temp = nodeX[i];
+          nodeX[i] = nodeX[k];
+          nodeX[k] = temp;
+        }
+      }
+    }
+
+    // Fill between pairs of nodes
+    for (int i = 0; i < nodes - 1; i += 2) {
+      int startX = nodeX[i];
+      int endX = nodeX[i + 1];
+
+      // Clip to screen
+      if (startX < 0) startX = 0;
+      if (endX >= getScreenWidth()) endX = getScreenWidth() - 1;
+
+      // Draw horizontal line
+      for (int x = startX; x <= endX; x++) {
+        drawPixel(x, scanY, state);
+      }
+    }
+  }
+
+  free(nodeX);
+}
+
 void GfxRenderer::clearScreen(const uint8_t color) const { einkDisplay.clearScreen(color); }
 
 void GfxRenderer::invertScreen() const {

lib/GfxRenderer/GfxRenderer.h

@@ -68,6 +68,8 @@ class GfxRenderer {
   void drawImage(const uint8_t bitmap[], int x, int y, int width, int height) const;
   void drawBitmap(const Bitmap& bitmap, int x, int y, int maxWidth, int maxHeight, float cropX = 0,
                   float cropY = 0) const;
+  void drawBitmap1Bit(const Bitmap& bitmap, int x, int y, int maxWidth, int maxHeight) const;
+  void fillPolygon(const int* xPoints, const int* yPoints, int numPoints, bool state = true) const;
 
   // Text
   int getTextWidth(int fontId, const char* text, EpdFontFamily::Style style = EpdFontFamily::REGULAR) const;
@@ -97,8 +99,8 @@ class GfxRenderer {
   void copyGrayscaleLsbBuffers() const;
   void copyGrayscaleMsbBuffers() const;
   void displayGrayBuffer() const;
-  bool storeBwBuffer();  // Returns true if buffer was stored successfully
-  void restoreBwBuffer();
+  bool storeBwBuffer();    // Returns true if buffer was stored successfully
+  void restoreBwBuffer();  // Restore and free the stored buffer
   void cleanupGrayscaleWithFrameBuffer() const;
 
   // Low level functions