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.h

@@ -28,6 +28,10 @@ class Txt {
   [[nodiscard]] bool generateCoverBmp() const;
   [[nodiscard]] std::string findCoverImage() const;
 
+  // Thumbnail paths (matching Epub/Xtc pattern for home screen covers)
+  [[nodiscard]] std::string getThumbBmpPath() const;
+  [[nodiscard]] std::string getThumbBmpPath(int height) const;
+
   // Read content from file
   [[nodiscard]] bool readContent(uint8_t* buffer, size_t offset, size_t length) const;
 };