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/parsers/ContentOpfParser.cpp

@@ -161,6 +161,7 @@ void XMLCALL ContentOpfParser::startElement(void* userData, const XML_Char* name
     std::string itemId;
     std::string href;
     std::string mediaType;
+    std::string properties;
 
     for (int i = 0; atts[i]; i += 2) {
       if (strcmp(atts[i], "id") == 0) {
@@ -169,6 +170,8 @@ void XMLCALL ContentOpfParser::startElement(void* userData, const XML_Char* name
         href = self->baseContentPath + atts[i + 1];
       } else if (strcmp(atts[i], "media-type") == 0) {
         mediaType = atts[i + 1];
+      } else if (strcmp(atts[i], "properties") == 0) {
+        properties = atts[i + 1];
       }
     }
 
@@ -188,6 +191,15 @@ void XMLCALL ContentOpfParser::startElement(void* userData, const XML_Char* name
                       href.c_str());
       }
     }
+
+    // EPUB 3: Check for nav document (properties contains "nav")
+    if (!properties.empty() && self->tocNavPath.empty()) {
+      // Properties is space-separated, check if "nav" is present as a word
+      if (properties == "nav" || properties.find("nav ") == 0 || properties.find(" nav") != std::string::npos) {
+        self->tocNavPath = href;
+        Serial.printf("[%lu] [COF] Found EPUB 3 nav document: %s\n", millis(), href.c_str());
+      }
+    }
     return;
   }