Generate styled placeholder covers (title, author, book icon) when a
book has no embedded cover image, instead of showing a blank rectangle.
- Add PlaceholderCoverGenerator lib with 1-bit BMP rendering, scaled
fonts, word-wrap, and a book icon bitmap
- Integrate as fallback in Epub/Xtc/Txt reader activities and
SleepActivity after format-specific cover generation fails
- Add fallback in HomeActivity::loadRecentCovers() so the home screen
also shows placeholder thumbnails when cache is cleared
- Add Txt::getThumbBmpPath() for TXT thumbnail support
- Add helper scripts for icon and layout preview generation
Co-authored-by: Cursor <cursoragent@cursor.com>
