perf: Improve large CSS files handling (#779)

## Summary

Closes #766. Thank you for the help @bramschulting!

**What is the goal of this PR?** 
- First and foremost, fix issue #766.
- Through working on that, I realized the current CSS parsing/loading
code can be improved dramatically for large files and still had
additional performance improvements to be made, even with EPUBs with
small CSS.

**What changes are included?**
- Stream CSS parsing and reuse normalization buffers to cut allocations
- Add rule limits and selector validation to release rules and free up
memory when needed
- Skip CSS parsing/loading entirely when "Book's Embedded Style" is off

## Additional Context

- My test EPUB has been updated
[here](https://github.com/jdk2pq/css-test-epub) to include a very large
CSS file to test this out

---

### 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**_, Codex

src/RecentBooksStore.cpp

@@ -88,7 +88,7 @@ RecentBook RecentBooksStore::getDataFromBook(std::string path) const {
   // If epub, try to load the metadata for title/author and cover
   if (StringUtils::checkFileExtension(lastBookFileName, ".epub")) {
     Epub epub(path, "/.crosspoint");
-    epub.load(false);
+    epub.load(false, true);
     return RecentBook{path, epub.getTitle(), epub.getAuthor(), epub.getThumbBmpPath()};
   } else if (StringUtils::checkFileExtension(lastBookFileName, ".xtch") ||
              StringUtils::checkFileExtension(lastBookFileName, ".xtc")) {