feat: Add PNG cover image support for EPUB books (#827)

## Summary
- EPUB books with PNG cover images now display covers on the home screen
instead of blank rectangles
- Adds `PngToBmpConverter` library mirroring the existing
`JpegToBmpConverter` pattern
- Uses miniz (already in the project) for streaming zlib decompression
of PNG IDAT data
- Supports all PNG color types (Grayscale, RGB, RGBA, Palette,
Gray+Alpha)
- Optimized for ESP32-C3: batch grayscale conversion, 2KB read buffer,
same area-averaging scaling and Atkinson dithering as the JPEG path

## Changes
- **New:** `lib/PngToBmpConverter/PngToBmpConverter.h` — Public API
matching JpegToBmpConverter's interface
- **New:** `lib/PngToBmpConverter/PngToBmpConverter.cpp` — Streaming PNG
decoder + BMP converter
- **Modified:** `lib/Epub/Epub.cpp` — Added `.png` handling in
`generateCoverBmp()` and `generateThumbBmp()`

## Test plan
- [x] Tested with EPUB files using PNG covers — covers appear correctly
on home screen
- [ ] Verify with various PNG color types (most stock EPUBs use 8-bit
RGB)
- [ ] Confirm no regressions with JPEG cover EPUBs

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

**New Features**
- Added PNG format support for EPUB cover and thumbnail images. PNG
files are automatically processed and cached alongside existing
supported formats. This enhancement enables users to leverage PNG cover
artwork when generating EPUB files, improving workflow flexibility and
compatibility with common image sources.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: Nik Outchcunis <outchy@gmail.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: Dave Allie <dave@daveallie.com>

lib/Epub/Epub/converters/JpegToFramebufferConverter.cpp

@@ -1,7 +1,7 @@
 #include "JpegToFramebufferConverter.h"
 
 #include <GfxRenderer.h>
-#include <HardwareSerial.h>
+#include <Logging.h>
 #include <SDCardManager.h>
 #include <SdFat.h>
 #include <picojpeg.h>
@@ -23,7 +23,7 @@ struct JpegContext {
 bool JpegToFramebufferConverter::getDimensionsStatic(const std::string& imagePath, ImageDimensions& out) {
   FsFile file;
   if (!Storage.openFileForRead("JPG", imagePath, file)) {
-    Serial.printf("[%lu] [JPG] Failed to open file for dimensions: %s\n", millis(), imagePath.c_str());
+    LOG_ERR("JPG", "Failed to open file for dimensions: %s", imagePath.c_str());
     return false;
   }
 
@@ -34,23 +34,23 @@ bool JpegToFramebufferConverter::getDimensionsStatic(const std::string& imagePat
   file.close();
 
   if (status != 0) {
-    Serial.printf("[%lu] [JPG] Failed to init JPEG for dimensions: %d\n", millis(), status);
+    LOG_ERR("JPG", "Failed to init JPEG for dimensions: %d", status);
     return false;
   }
 
   out.width = imageInfo.m_width;
   out.height = imageInfo.m_height;
-  Serial.printf("[%lu] [JPG] Image dimensions: %dx%d\n", millis(), out.width, out.height);
+  LOG_DBG("JPG", "Image dimensions: %dx%d", out.width, out.height);
   return true;
 }
 
 bool JpegToFramebufferConverter::decodeToFramebuffer(const std::string& imagePath, GfxRenderer& renderer,
                                                      const RenderConfig& config) {
-  Serial.printf("[%lu] [JPG] Decoding JPEG: %s\n", millis(), imagePath.c_str());
+  LOG_DBG("JPG", "Decoding JPEG: %s", imagePath.c_str());
 
   FsFile file;
   if (!Storage.openFileForRead("JPG", imagePath, file)) {
-    Serial.printf("[%lu] [JPG] Failed to open file: %s\n", millis(), imagePath.c_str());
+    LOG_ERR("JPG", "Failed to open file: %s", imagePath.c_str());
     return false;
   }
 
@@ -59,7 +59,7 @@ bool JpegToFramebufferConverter::decodeToFramebuffer(const std::string& imagePat
 
   int status = pjpeg_decode_init(&imageInfo, jpegReadCallback, &context, 0);
   if (status != 0) {
-    Serial.printf("[%lu] [JPG] picojpeg init failed: %d\n", millis(), status);
+    LOG_ERR("JPG", "picojpeg init failed: %d", status);
     file.close();
     return false;
   }
@@ -93,12 +93,11 @@ bool JpegToFramebufferConverter::decodeToFramebuffer(const std::string& imagePat
     destHeight = (int)(imageInfo.m_height * scale);
   }
 
-  Serial.printf("[%lu] [JPG] JPEG %dx%d -> %dx%d (scale %.2f), scan type: %d, MCU: %dx%d\n", millis(),
-                imageInfo.m_width, imageInfo.m_height, destWidth, destHeight, scale, imageInfo.m_scanType,
-                imageInfo.m_MCUWidth, imageInfo.m_MCUHeight);
+  LOG_DBG("JPG", "JPEG %dx%d -> %dx%d (scale %.2f), scan type: %d, MCU: %dx%d", imageInfo.m_width, imageInfo.m_height,
+          destWidth, destHeight, scale, imageInfo.m_scanType, imageInfo.m_MCUWidth, imageInfo.m_MCUHeight);
 
   if (!imageInfo.m_pMCUBufR || !imageInfo.m_pMCUBufG || !imageInfo.m_pMCUBufB) {
-    Serial.printf("[%lu] [JPG] Null buffer pointers in imageInfo\n", millis());
+    LOG_ERR("JPG", "Null buffer pointers in imageInfo");
     file.close();
     return false;
   }
@@ -111,7 +110,7 @@ bool JpegToFramebufferConverter::decodeToFramebuffer(const std::string& imagePat
   bool caching = !config.cachePath.empty();
   if (caching) {
     if (!cache.allocate(destWidth, destHeight, config.x, config.y)) {
-      Serial.printf("[%lu] [JPG] Failed to allocate cache buffer, continuing without caching\n", millis());
+      LOG_ERR("JPG", "Failed to allocate cache buffer, continuing without caching");
       caching = false;
     }
   }
@@ -125,7 +124,7 @@ bool JpegToFramebufferConverter::decodeToFramebuffer(const std::string& imagePat
       break;
     }
     if (status != 0) {
-      Serial.printf("[%lu] [JPG] MCU decode failed: %d\n", millis(), status);
+      LOG_ERR("JPG", "MCU decode failed: %d", status);
       file.close();
       return false;
     }
@@ -254,7 +253,7 @@ bool JpegToFramebufferConverter::decodeToFramebuffer(const std::string& imagePat
     }
   }
 
-  Serial.printf("[%lu] [JPG] Decoding complete\n", millis());
+  LOG_DBG("JPG", "Decoding complete");
   file.close();
 
   // Write cache file if caching was enabled