feat: Add EPUB embedded image support (JPEG/PNG)

Cherry-pick merge from pablohc/crosspoint-reader@2d8cbcf, based on
upstream PR #556 by martinbrook with pablohc's refresh optimization.

- Add JPEG decoder (picojpeg) and PNG decoder (PNGdec) with 4-level
  grayscale Bayer dithering for e-ink display
- Add pixel caching system (.pxc files) for fast image re-rendering
- Integrate image extraction from EPUB HTML parser (<img> tag support)
- Add ImageBlock/PageImage types with serialization support
- Add image-aware refresh optimization (double FAST_REFRESH technique)
- Add experimental displayWindow() partial refresh support
- Bump section cache version 12->13 to invalidate stale caches
- Resolve TAG_PageImage=3 to avoid conflict with mod's TAG_PageTableRow=2

Co-authored-by: Cursor <cursoragent@cursor.com>

platformio.ini

@@ -30,6 +30,9 @@ build_flags =
   -std=c++2a
 # Enable UTF-8 long file names in SdFat
   -DUSE_UTF8_LONG_NAMES=1
+# Increase PNG scanline buffer to support up to 800px wide images
+# Default is (320*4+1)*2=2562, we need more for larger images
+  -DPNG_MAX_BUFFERED_PIXELS=6402
 
 ; Board configuration
 board_build.flash_mode = dio
@@ -47,6 +50,7 @@ lib_deps =
   SDCardManager=symlink://open-x4-sdk/libs/hardware/SDCardManager
   bblanchon/ArduinoJson @ 7.4.2
   ricmoo/QRCode @ 0.0.1
+  bitbank2/PNGdec @ ^1.0.0
   links2004/WebSockets @ 2.7.3
 
 [env:default]