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/PngToFramebufferConverter.cpp

@@ -1,7 +1,7 @@
 #include "PngToFramebufferConverter.h"
 
 #include <GfxRenderer.h>
-#include <HardwareSerial.h>
+#include <Logging.h>
 #include <PNGdec.h>
 #include <SDCardManager.h>
 #include <SdFat.h>
@@ -216,14 +216,13 @@ int pngDrawCallback(PNGDRAW* pDraw) {
 bool PngToFramebufferConverter::getDimensionsStatic(const std::string& imagePath, ImageDimensions& out) {
   size_t freeHeap = ESP.getFreeHeap();
   if (freeHeap < MIN_FREE_HEAP_FOR_PNG) {
-    Serial.printf("[%lu] [PNG] Not enough heap for PNG decoder (%u free, need %u)\n", millis(), freeHeap,
-                  MIN_FREE_HEAP_FOR_PNG);
+    LOG_ERR("PNG", "Not enough heap for PNG decoder (%u free, need %u)", freeHeap, MIN_FREE_HEAP_FOR_PNG);
     return false;
   }
 
   PNG* png = new (std::nothrow) PNG();
   if (!png) {
-    Serial.printf("[%lu] [PNG] Failed to allocate PNG decoder for dimensions\n", millis());
+    LOG_ERR("PNG", "Failed to allocate PNG decoder for dimensions");
     return false;
   }
 
@@ -231,7 +230,7 @@ bool PngToFramebufferConverter::getDimensionsStatic(const std::string& imagePath
                      nullptr);
 
   if (rc != 0) {
-    Serial.printf("[%lu] [PNG] Failed to open PNG for dimensions: %d\n", millis(), rc);
+    LOG_ERR("PNG", "Failed to open PNG for dimensions: %d", rc);
     delete png;
     return false;
   }
@@ -246,19 +245,18 @@ bool PngToFramebufferConverter::getDimensionsStatic(const std::string& imagePath
 
 bool PngToFramebufferConverter::decodeToFramebuffer(const std::string& imagePath, GfxRenderer& renderer,
                                                     const RenderConfig& config) {
-  Serial.printf("[%lu] [PNG] Decoding PNG: %s\n", millis(), imagePath.c_str());
+  LOG_DBG("PNG", "Decoding PNG: %s", imagePath.c_str());
 
   size_t freeHeap = ESP.getFreeHeap();
   if (freeHeap < MIN_FREE_HEAP_FOR_PNG) {
-    Serial.printf("[%lu] [PNG] Not enough heap for PNG decoder (%u free, need %u)\n", millis(), freeHeap,
-                  MIN_FREE_HEAP_FOR_PNG);
+    LOG_ERR("PNG", "Not enough heap for PNG decoder (%u free, need %u)", freeHeap, MIN_FREE_HEAP_FOR_PNG);
     return false;
   }
 
   // Heap-allocate PNG decoder (~42 KB) - freed at end of function
   PNG* png = new (std::nothrow) PNG();
   if (!png) {
-    Serial.printf("[%lu] [PNG] Failed to allocate PNG decoder\n", millis());
+    LOG_ERR("PNG", "Failed to allocate PNG decoder");
     return false;
   }
 
@@ -271,7 +269,7 @@ bool PngToFramebufferConverter::decodeToFramebuffer(const std::string& imagePath
   int rc = png->open(imagePath.c_str(), pngOpenWithHandle, pngCloseWithHandle, pngReadWithHandle, pngSeekWithHandle,
                      pngDrawCallback);
   if (rc != PNG_SUCCESS) {
-    Serial.printf("[%lu] [PNG] Failed to open PNG: %d\n", millis(), rc);
+    LOG_ERR("PNG", "Failed to open PNG: %d", rc);
     delete png;
     return false;
   }
@@ -303,8 +301,8 @@ bool PngToFramebufferConverter::decodeToFramebuffer(const std::string& imagePath
   }
   ctx.lastDstY = -1;  // Reset row tracking
 
-  Serial.printf("[%lu] [PNG] PNG %dx%d -> %dx%d (scale %.2f), bpp: %d\n", millis(), ctx.srcWidth, ctx.srcHeight,
-                ctx.dstWidth, ctx.dstHeight, ctx.scale, png->getBpp());
+  LOG_DBG("PNG", "PNG %dx%d -> %dx%d (scale %.2f), bpp: %d", ctx.srcWidth, ctx.srcHeight, ctx.dstWidth, ctx.dstHeight,
+          ctx.scale, png->getBpp());
 
   if (png->getBpp() != 8) {
     warnUnsupportedFeature("bit depth (" + std::to_string(png->getBpp()) + "bpp)", imagePath);
@@ -314,7 +312,7 @@ bool PngToFramebufferConverter::decodeToFramebuffer(const std::string& imagePath
   const size_t grayBufSize = PNG_MAX_BUFFERED_PIXELS / 2;
   ctx.grayLineBuffer = static_cast<uint8_t*>(malloc(grayBufSize));
   if (!ctx.grayLineBuffer) {
-    Serial.printf("[%lu] [PNG] Failed to allocate gray line buffer\n", millis());
+    LOG_ERR("PNG", "Failed to allocate gray line buffer");
     png->close();
     delete png;
     return false;
@@ -324,7 +322,7 @@ bool PngToFramebufferConverter::decodeToFramebuffer(const std::string& imagePath
   ctx.caching = !config.cachePath.empty();
   if (ctx.caching) {
     if (!ctx.cache.allocate(ctx.dstWidth, ctx.dstHeight, config.x, config.y)) {
-      Serial.printf("[%lu] [PNG] Failed to allocate cache buffer, continuing without caching\n", millis());
+      LOG_ERR("PNG", "Failed to allocate cache buffer, continuing without caching");
       ctx.caching = false;
     }
   }
@@ -337,7 +335,7 @@ bool PngToFramebufferConverter::decodeToFramebuffer(const std::string& imagePath
   ctx.grayLineBuffer = nullptr;
 
   if (rc != PNG_SUCCESS) {
-    Serial.printf("[%lu] [PNG] Decode failed: %d\n", millis(), rc);
+    LOG_ERR("PNG", "Decode failed: %d", rc);
     png->close();
     delete png;
     return false;
@@ -345,7 +343,7 @@ bool PngToFramebufferConverter::decodeToFramebuffer(const std::string& imagePath
 
   png->close();
   delete png;
-  Serial.printf("[%lu] [PNG] PNG decoding complete - render time: %lu ms\n", millis(), decodeTime);
+  LOG_DBG("PNG", "PNG decoding complete - render time: %lu ms", decodeTime);
 
   // Write cache file if caching was enabled and buffer was allocated
   if (ctx.caching) {