fix: Handle EPUB 3 TOC to spine mapping when nav file in subdirectory (#332)

## Summary

- Nav file in EPUB 3 file is a HTML file with relative hrefs
- If this file exists anywhere but in the same location as the
content.opf file, navigating in the book will fail
- Bump the book cache version to rebuild potentially broken books

## Additional Context

- Fixes https://github.com/daveallie/crosspoint-reader/issues/264

---

### AI Usage

While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it
helps set the right context for reviewers.

Did you use AI tools to help write this code?

- [ ] Yes
- [ ] Partially
- [x] No

lib/Epub/Epub.cpp

@@ -167,7 +167,10 @@ bool Epub::parseTocNavFile() const {
   }
   const auto navSize = tempNavFile.size();
 
-  TocNavParser navParser(contentBasePath, navSize, bookMetadataCache.get());
+  // Note: We can't use `contentBasePath` here as the nav file may be in a different folder to the content.opf
+  // and the HTMLX nav file will have hrefs relative to itself
+  const std::string navContentBasePath = tocNavItem.substr(0, tocNavItem.find_last_of('/') + 1);
+  TocNavParser navParser(navContentBasePath, navSize, bookMetadataCache.get());
 
   if (!navParser.setup()) {
     Serial.printf("[%lu] [EBP] Could not setup toc nav parser\n", millis());