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

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### AI Usage

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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/BookMetadataCache.cpp

@@ -9,7 +9,7 @@
 #include "FsHelpers.h"
 
 namespace {
-constexpr uint8_t BOOK_CACHE_VERSION = 3;
+constexpr uint8_t BOOK_CACHE_VERSION = 4;
 constexpr char bookBinFile[] = "/book.bin";
 constexpr char tmpSpineBinFile[] = "/spine.bin.tmp";
 constexpr char tmpTocBinFile[] = "/toc.bin.tmp";