fix: dynamic continue reading card sizing based on cover image

- Adapt card width to cover image aspect ratio when available
- Maintain fixed 400px height for consistent layout
- Use actual image dimensions instead of hardcoded 240px width for EPUBs
- Add fallback to half-screen size when no cover image exists

lib/Epub/Epub.cpp

@@ -468,10 +468,47 @@ bool Epub::generateThumbBmp() const {
       coverJpg.close();
       return false;
     }
-    // Use smaller target size for Continue Reading card (half of screen: 240x400)
-    // Generate 1-bit BMP for fast home screen rendering (no gray passes needed)
-    constexpr int THUMB_TARGET_WIDTH = 240;
-    constexpr int THUMB_TARGET_HEIGHT = 400;
+
+    // Generate 1-bit BMP maintaining aspect ratio with fixed height of 400px
+    // This ensures the card width adapts to the actual cover image aspect ratio
+    const int THUMB_TARGET_HEIGHT = 400;
+
+    // Read a small portion of the JPG file to determine its dimensions
+    uint8_t headerBuffer[200];
+    size_t bytesRead = coverJpg.read(headerBuffer, sizeof(headerBuffer));
+    coverJpg.seek(0);  // Reset file position
+
+    int imgWidth = 240, imgHeight = 400;  // Default fallback
+
+    // Simple header parsing to get image dimensions (works for most JPGs)
+    if (bytesRead > 0) {
+      // Look for SOF (Start of Frame) markers
+      for (size_t i = 0; i < bytesRead - 10; i++) {
+        if (headerBuffer[i] == 0xFF && headerBuffer[i + 1] == 0xC0) {
+          // Found SOF0 marker
+          if (i + 7 < bytesRead) {
+            imgHeight = (headerBuffer[i + 5] << 8) | headerBuffer[i + 6];
+            imgWidth = (headerBuffer[i + 7] << 8) | headerBuffer[i + 8];
+            break;
+          }
+        } else if (headerBuffer[i] == 0xFF && headerBuffer[i + 1] == 0xC2) {
+          // Found SOF2 marker
+          if (i + 7 < bytesRead) {
+            imgHeight = (headerBuffer[i + 5] << 8) | headerBuffer[i + 6];
+            imgWidth = (headerBuffer[i + 7] << 8) | headerBuffer[i + 8];
+            break;
+          }
+        }
+      }
+    }
+
+    // Calculate target width maintaining aspect ratio
+    const float aspectRatio = static_cast<float>(imgWidth) / static_cast<float>(imgHeight);
+    const int THUMB_TARGET_WIDTH = static_cast<int>(THUMB_TARGET_HEIGHT * aspectRatio);
+
+    Serial.printf("[%lu] [EBP] Original JPG: %dx%d, Target: %dx%d\n", millis(), imgWidth, imgHeight, THUMB_TARGET_WIDTH,
+                  THUMB_TARGET_HEIGHT);
+
     const bool success = JpegToBmpConverter::jpegFileTo1BitBmpStreamWithSize(coverJpg, thumbBmp, THUMB_TARGET_WIDTH,
                                                                              THUMB_TARGET_HEIGHT);
     coverJpg.close();

lib/JpegToBmpConverter/JpegToBmpConverter.cpp

@@ -567,5 +567,5 @@ bool JpegToBmpConverter::jpegFileToBmpStreamWithSize(FsFile& jpegFile, Print& bm
 // Convert to 1-bit BMP (black and white only, no grays) for fast home screen rendering
 bool JpegToBmpConverter::jpegFileTo1BitBmpStreamWithSize(FsFile& jpegFile, Print& bmpOut, int targetMaxWidth,
                                                          int targetMaxHeight) {
-  return jpegFileToBmpStreamInternal(jpegFile, bmpOut, targetMaxWidth, targetMaxHeight, true);
+  return jpegFileToBmpStreamInternal(jpegFile, bmpOut, targetMaxWidth, targetMaxHeight, true, false);
 }

src/activities/home/HomeActivity.cpp

@@ -223,9 +223,50 @@ void HomeActivity::render() {
   constexpr int bottomMargin = 60;
 
   // --- Top "book" card for the current title (selectorIndex == 0) ---
-  const int bookWidth = pageWidth / 2;
-  const int bookHeight = pageHeight / 2;
-  const int bookX = (pageWidth - bookWidth) / 2;
+  // When there's no cover image, use fixed size (half screen)
+  // When there's cover image, adapt width to image aspect ratio, keep height fixed at 400px
+  const int baseHeight = 400;  // Fixed height for both scenarios
+
+  int bookWidth, bookX;
+  if (hasCoverImage) {
+    // When there's cover, calculate width based on image aspect ratio
+    // Use default width ratio as fallback if we can't get image dimensions
+    const float defaultWidthRatio = 0.5f;  // Same as half screen
+    bookWidth = static_cast<int>(pageWidth * defaultWidthRatio);
+
+    // If we have cover bitmap path, try to get actual image dimensions
+    if (!coverBmpPath.empty()) {
+      FsFile file;
+      if (SdMan.openFileForRead("HOME", coverBmpPath, file)) {
+        Bitmap bitmap(file);
+        if (bitmap.parseHeaders() == BmpReaderError::Ok) {
+          const int imgWidth = bitmap.getWidth();
+          const int imgHeight = bitmap.getHeight();
+
+          // Calculate width based on aspect ratio, maintaining 400px height
+          if (imgHeight > 0) {
+            const float aspectRatio = static_cast<float>(imgWidth) / static_cast<float>(imgHeight);
+            bookWidth = static_cast<int>(baseHeight * aspectRatio);
+
+            // Ensure width doesn't exceed reasonable limits (max 90% of screen width)
+            const int maxWidth = static_cast<int>(pageWidth * 0.9f);
+            if (bookWidth > maxWidth) {
+              bookWidth = maxWidth;
+            }
+          }
+        }
+        file.close();
+      }
+    }
+
+    bookX = (pageWidth - bookWidth) / 2;
+  } else {
+    // No cover: use half screen size
+    bookWidth = pageWidth / 2;
+    bookX = (pageWidth - bookWidth) / 2;
+  }
+
+  const int bookHeight = baseHeight;
   constexpr int bookY = 30;
   const bool bookSelected = hasContinueReading && selectorIndex == 0;