Add EPUB 3 nav.xhtml TOC support (#197)

## Summary

* **What is the goal of this PR?** Add EPUB 3 support by implementing
native navigation document (nav.xhtml) parsing with NCX fallback,
addressing issue Fixes: #143.

  * **What changes are included?**
- New `TocNavParser` for parsing EPUB 3 HTML5 navigation documents
(`<nav epub:type="toc">`)
- Detection of nav documents via `properties="nav"` attribute in OPF
manifest
- Fallback logic: try EPUB 3 nav first, fall back to NCX (EPUB 2) if
unavailable
- Graceful degradation: books without any TOC now load with a warning
instead of failing

  ## Additional Context

* The implementation follows the existing streaming XML parser pattern
using Expat to minimize RAM usage on the ESP32-C3
* EPUB 3 books that include both nav.xhtml and toc.ncx will prefer the
nav document (per EPUB 3 spec recommendation)
* No breaking changes - existing EPUB 2 books continue to work as before
* Tested on examples from
https://idpf.github.io/epub3-samples/30/samples.html

lib/Epub/Epub.h

@@ -12,8 +12,10 @@
 class ZipFile;
 
 class Epub {
-  // the ncx file
+  // the ncx file (EPUB 2)
   std::string tocNcxItem;
+  // the nav file (EPUB 3)
+  std::string tocNavItem;
   // where is the EPUBfile?
   std::string filepath;
   // the base path for items in the EPUB file
@@ -26,6 +28,7 @@ class Epub {
   bool findContentOpfFile(std::string* contentOpfFile) const;
   bool parseContentOpf(BookMetadataCache::BookMetadata& bookMetadata);
   bool parseTocNcxFile() const;
+  bool parseTocNavFile() const;
 
  public:
   explicit Epub(std::string filepath, const std::string& cacheDir) : filepath(std::move(filepath)) {