6aa0b865c2
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>