feat(sleep): pre-calculate EPUB cover dimensions for FIT/CROP modes

- Add JpegToBmpConverter::getJpegDimensions() to extract JPEG dimensions
- Fix getCoverBmpPath() ternary operator precedence bug
- Pre-calculate exact dimensions per mode:
  * FIT: width=480px, height=(480*jpegHeight)/jpegWidth
  * CROP: height=800px, width=(800*jpegWidth)/jpegHeight
- Generate separate cache files (cover_fit.bmp vs cover_crop.bmp)
- Eliminates runtime scaling artifacts, improves sleep screen clarity

lib/JpegToBmpConverter/JpegToBmpConverter.cpp

@@ -565,3 +565,32 @@ bool JpegToBmpConverter::jpegFileTo1BitBmpStreamWithSize(FsFile& jpegFile, Print
                                                          int targetMaxHeight) {
   return jpegFileToBmpStreamInternal(jpegFile, bmpOut, targetMaxWidth, targetMaxHeight, true);
 }
+
+// Get JPEG dimensions without full conversion
+bool JpegToBmpConverter::getJpegDimensions(FsFile& jpegFile, int& width, int& height) {
+  // Reset file position to beginning
+  if (!jpegFile.seek(0)) {
+    Serial.printf("[%lu] [JPG] Failed to seek to beginning of JPEG file\n", millis());
+    return false;
+  }
+
+  // Initialize JPEG decoder
+  pjpeg_image_info_t imageInfo = {};
+  JpegReadContext context{jpegFile, {}, 0, 0};
+
+  const int decodeStatus = pjpeg_decode_init(&imageInfo, jpegReadCallback, &context, false);
+  if (decodeStatus != 0) {
+    Serial.printf("[%lu] [JPG] pjpeg_decode_init failed with status %d\n", millis(), decodeStatus);
+    return false;
+  }
+
+  // Get dimensions from image info
+  width = imageInfo.m_width;
+  height = imageInfo.m_height;
+
+  Serial.printf("[%lu] [JPG] Read JPEG dimensions: %dx%d\n", millis(), width, height);
+
+  // Reset file position after reading
+  jpegFile.seek(0);
+  return true;
+}

lib/JpegToBmpConverter/JpegToBmpConverter.h

@@ -16,4 +16,6 @@ class JpegToBmpConverter {
   static bool jpegFileToBmpStreamWithSize(FsFile& jpegFile, Print& bmpOut, int targetMaxWidth, int targetMaxHeight);
   // Convert to 1-bit BMP (black and white only, no grays) for fast home screen rendering
   static bool jpegFileTo1BitBmpStreamWithSize(FsFile& jpegFile, Print& bmpOut, int targetMaxWidth, int targetMaxHeight);
+  // Get JPEG dimensions without full conversion
+  static bool getJpegDimensions(FsFile& jpegFile, int& width, int& height);
 };