feat: Add placeholder cover generator for books without covers

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>

lib/Txt/Txt.cpp

@@ -97,6 +97,9 @@ std::string Txt::findCoverImage() const {
 
 std::string Txt::getCoverBmpPath() const { return cachePath + "/cover.bmp"; }
 
+std::string Txt::getThumbBmpPath() const { return cachePath + "/thumb_[HEIGHT].bmp"; }
+std::string Txt::getThumbBmpPath(int height) const { return cachePath + "/thumb_" + std::to_string(height) + ".bmp"; }
+
 bool Txt::generateCoverBmp() const {
   // Already generated, return true
   if (Storage.exists(getCoverBmpPath().c_str())) {