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/blocks/ImageBlock.cpp

@@ -1,8 +1,7 @@
 #include "ImageBlock.h"
 
-#include <FsHelpers.h>
 #include <GfxRenderer.h>
-#include <HardwareSerial.h>
+#include <Logging.h>
 #include <SDCardManager.h>
 #include <Serialization.h>
 
@@ -47,8 +46,8 @@ bool renderFromCache(GfxRenderer& renderer, const std::string& cachePath, int x,
   int widthDiff = abs(cachedWidth - expectedWidth);
   int heightDiff = abs(cachedHeight - expectedHeight);
   if (widthDiff > 1 || heightDiff > 1) {
-    Serial.printf("[%lu] [IMG] Cache dimension mismatch: %dx%d vs %dx%d\n", millis(), cachedWidth, cachedHeight,
-                  expectedWidth, expectedHeight);
+    LOG_ERR("IMG", "Cache dimension mismatch: %dx%d vs %dx%d", cachedWidth, cachedHeight, expectedWidth,
+            expectedHeight);
     cacheFile.close();
     return false;
   }
@@ -57,20 +56,20 @@ bool renderFromCache(GfxRenderer& renderer, const std::string& cachePath, int x,
   expectedWidth = cachedWidth;
   expectedHeight = cachedHeight;
 
-  Serial.printf("[%lu] [IMG] Loading from cache: %s (%dx%d)\n", millis(), cachePath.c_str(), cachedWidth, cachedHeight);
+  LOG_DBG("IMG", "Loading from cache: %s (%dx%d)", cachePath.c_str(), cachedWidth, cachedHeight);
 
   // Read and render row by row to minimize memory usage
   const int bytesPerRow = (cachedWidth + 3) / 4;  // 2 bits per pixel, 4 pixels per byte
   uint8_t* rowBuffer = (uint8_t*)malloc(bytesPerRow);
   if (!rowBuffer) {
-    Serial.printf("[%lu] [IMG] Failed to allocate row buffer\n", millis());
+    LOG_ERR("IMG", "Failed to allocate row buffer");
     cacheFile.close();
     return false;
   }
 
   for (int row = 0; row < cachedHeight; row++) {
     if (cacheFile.read(rowBuffer, bytesPerRow) != bytesPerRow) {
-      Serial.printf("[%lu] [IMG] Cache read error at row %d\n", millis(), row);
+      LOG_ERR("IMG", "Cache read error at row %d", row);
       free(rowBuffer);
       cacheFile.close();
       return false;
@@ -88,22 +87,22 @@ bool renderFromCache(GfxRenderer& renderer, const std::string& cachePath, int x,
 
   free(rowBuffer);
   cacheFile.close();
-  Serial.printf("[%lu] [IMG] Cache render complete\n", millis());
+  LOG_DBG("IMG", "Cache render complete");
   return true;
 }
 
 }  // namespace
 
 void ImageBlock::render(GfxRenderer& renderer, const int x, const int y) {
-  Serial.printf("[%lu] [IMG] Rendering image at %d,%d: %s (%dx%d)\n", millis(), x, y, imagePath.c_str(), width, height);
+  LOG_DBG("IMG", "Rendering image at %d,%d: %s (%dx%d)", x, y, imagePath.c_str(), width, height);
 
   const int screenWidth = renderer.getScreenWidth();
   const int screenHeight = renderer.getScreenHeight();
 
   // Bounds check render position using logical screen dimensions
   if (x < 0 || y < 0 || x + width > screenWidth || y + height > screenHeight) {
-    Serial.printf("[%lu] [IMG] Invalid render position: (%d,%d) size (%dx%d) screen (%dx%d)\n", millis(), x, y, width,
-                  height, screenWidth, screenHeight);
+    LOG_ERR("IMG", "Invalid render position: (%d,%d) size (%dx%d) screen (%dx%d)", x, y, width, height, screenWidth,
+            screenHeight);
     return;
   }
 
@@ -117,18 +116,18 @@ void ImageBlock::render(GfxRenderer& renderer, const int x, const int y) {
   // Check if image file exists
   FsFile file;
   if (!Storage.openFileForRead("IMG", imagePath, file)) {
-    Serial.printf("[%lu] [IMG] Image file not found: %s\n", millis(), imagePath.c_str());
+    LOG_ERR("IMG", "Image file not found: %s", imagePath.c_str());
     return;
   }
   size_t fileSize = file.size();
   file.close();
 
   if (fileSize == 0) {
-    Serial.printf("[%lu] [IMG] Image file is empty: %s\n", millis(), imagePath.c_str());
+    LOG_ERR("IMG", "Image file is empty: %s", imagePath.c_str());
     return;
   }
 
-  Serial.printf("[%lu] [IMG] Decoding and caching: %s\n", millis(), imagePath.c_str());
+  LOG_DBG("IMG", "Decoding and caching: %s", imagePath.c_str());
 
   RenderConfig config;
   config.x = x;
@@ -143,19 +142,19 @@ void ImageBlock::render(GfxRenderer& renderer, const int x, const int y) {
 
   ImageToFramebufferDecoder* decoder = ImageDecoderFactory::getDecoder(imagePath);
   if (!decoder) {
-    Serial.printf("[%lu] [IMG] No decoder found for image: %s\n", millis(), imagePath.c_str());
+    LOG_ERR("IMG", "No decoder found for image: %s", imagePath.c_str());
     return;
   }
 
-  Serial.printf("[%lu] [IMG] Using %s decoder\n", millis(), decoder->getFormatName());
+  LOG_DBG("IMG", "Using %s decoder", decoder->getFormatName());
 
   bool success = decoder->decodeToFramebuffer(imagePath, renderer, config);
   if (!success) {
-    Serial.printf("[%lu] [IMG] Failed to decode image: %s\n", millis(), imagePath.c_str());
+    LOG_ERR("IMG", "Failed to decode image: %s", imagePath.c_str());
     return;
   }
 
-  Serial.printf("[%lu] [IMG] Decode successful\n", millis());
+  LOG_DBG("IMG", "Decode successful");
 }
 
 bool ImageBlock::serialize(FsFile& file) {