fix(epub): NFC-normalize EPUB text so NFD diacritics render correctly (#2277)

Co-authored-by: Uri Tauber <uritaube@gmail.com>
Co-authored-by: Julia <julia@uxj.io>

lib/Utf8/Utf8.h

@@ -12,6 +12,12 @@ size_t utf8RemoveLastChar(std::string& str);
 // Truncate string by removing N UTF-8 codepoints from the end.
 void utf8TruncateChars(std::string& str, size_t numChars);
 
+// Canonical composition (NFC) for the Latin / Vietnamese range: precomposes a
+// base letter followed by combining diacritical mark(s) into a single codepoint.
+// Needed because the device fonts have no combining-mark positioning, so text
+// stored in NFD (e.g. some EPUB chapter titles) otherwise renders broken.
+std::string utf8ComposeNfc(const std::string& in);
+
 // Truncate a raw char buffer to the last complete UTF-8 codepoint boundary.
 // Returns the new length (<= len). If the buffer ends mid-sequence, the
 // incomplete trailing bytes are excluded.