Introduce BookSettings utility for per-book settings stored in
the book's cache directory (book_settings.bin). Add "Letterbox Fill"
option to the EPUB reader menu that cycles Default/Dithered/Solid/None.
At sleep time, the per-book override is loaded and takes precedence
over the global setting for all book types (EPUB, XTC, TXT).
Co-authored-by: Cursor <cursoragent@cursor.com>
