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.cpp

@@ -4,6 +4,7 @@
 #include <HalStorage.h>
 #include <JpegToBmpConverter.h>
 #include <Logging.h>
+#include <PngToBmpConverter.h>
 #include <ZipFile.h>
 
 #include "Epub/parsers/ContainerParser.h"
@@ -486,12 +487,44 @@ bool Epub::generateCoverBmp(bool cropped) const {
       LOG_ERR("EBP", "Failed to generate BMP from cover image");
       Storage.remove(getCoverBmpPath(cropped).c_str());
     }
-    LOG_DBG("EBP", "Generated BMP from cover image, success: %s", success ? "yes" : "no");
+    LOG_DBG("EBP", "Generated BMP from JPG cover image, success: %s", success ? "yes" : "no");
     return success;
-  } else {
-    LOG_ERR("EBP", "Cover image is not a supported format, skipping");
   }
 
+  if (coverImageHref.substr(coverImageHref.length() - 4) == ".png") {
+    LOG_DBG("EBP", "Generating BMP from PNG cover image (%s mode)", cropped ? "cropped" : "fit");
+    const auto coverPngTempPath = getCachePath() + "/.cover.png";
+
+    FsFile coverPng;
+    if (!Storage.openFileForWrite("EBP", coverPngTempPath, coverPng)) {
+      return false;
+    }
+    readItemContentsToStream(coverImageHref, coverPng, 1024);
+    coverPng.close();
+
+    if (!Storage.openFileForRead("EBP", coverPngTempPath, coverPng)) {
+      return false;
+    }
+
+    FsFile coverBmp;
+    if (!Storage.openFileForWrite("EBP", getCoverBmpPath(cropped), coverBmp)) {
+      coverPng.close();
+      return false;
+    }
+    const bool success = PngToBmpConverter::pngFileToBmpStream(coverPng, coverBmp, cropped);
+    coverPng.close();
+    coverBmp.close();
+    Storage.remove(coverPngTempPath.c_str());
+
+    if (!success) {
+      LOG_ERR("EBP", "Failed to generate BMP from PNG cover image");
+      Storage.remove(getCoverBmpPath(cropped).c_str());
+    }
+    LOG_DBG("EBP", "Generated BMP from PNG cover image, success: %s", success ? "yes" : "no");
+    return success;
+  }
+
+  LOG_ERR("EBP", "Cover image is not a supported format, skipping");
   return false;
 }
 
@@ -549,6 +582,40 @@ bool Epub::generateThumbBmp(int height) const {
     }
     LOG_DBG("EBP", "Generated thumb BMP from JPG cover image, success: %s", success ? "yes" : "no");
     return success;
+  } else if (coverImageHref.substr(coverImageHref.length() - 4) == ".png") {
+    LOG_DBG("EBP", "Generating thumb BMP from PNG cover image");
+    const auto coverPngTempPath = getCachePath() + "/.cover.png";
+
+    FsFile coverPng;
+    if (!Storage.openFileForWrite("EBP", coverPngTempPath, coverPng)) {
+      return false;
+    }
+    readItemContentsToStream(coverImageHref, coverPng, 1024);
+    coverPng.close();
+
+    if (!Storage.openFileForRead("EBP", coverPngTempPath, coverPng)) {
+      return false;
+    }
+
+    FsFile thumbBmp;
+    if (!Storage.openFileForWrite("EBP", getThumbBmpPath(height), thumbBmp)) {
+      coverPng.close();
+      return false;
+    }
+    int THUMB_TARGET_WIDTH = height * 0.6;
+    int THUMB_TARGET_HEIGHT = height;
+    const bool success =
+        PngToBmpConverter::pngFileTo1BitBmpStreamWithSize(coverPng, thumbBmp, THUMB_TARGET_WIDTH, THUMB_TARGET_HEIGHT);
+    coverPng.close();
+    thumbBmp.close();
+    Storage.remove(coverPngTempPath.c_str());
+
+    if (!success) {
+      LOG_ERR("EBP", "Failed to generate thumb BMP from PNG cover image");
+      Storage.remove(getThumbBmpPath(height).c_str());
+    }
+    LOG_DBG("EBP", "Generated thumb BMP from PNG cover image, success: %s", success ? "yes" : "no");
+    return success;
   } else {
     LOG_ERR("EBP", "Cover image is not a supported format, skipping thumbnail");
   }