fix: Fix cover thumbnail pipeline for home screen

Remove empty sentinel BMP file from generateThumbBmp() that blocked
placeholder generation for books without covers. Add removeBook() to
RecentBooksStore and clear book from recents on cache delete. Ensure
home screen always generates placeholder when thumbnail generation fails.

Co-authored-by: Cursor <cursoragent@cursor.com>

lib/Epub/Epub.cpp

@@ -570,10 +570,6 @@ bool Epub::generateThumbBmp(int height) const {
     LOG_ERR("EBP", "Cover image is not a supported format, skipping thumbnail");
   }
 
-  // Write an empty bmp file to avoid generation attempts in the future
-  FsFile thumbBmp;
-  Storage.openFileForWrite("EBP", getThumbBmpPath(height), thumbBmp);
-  thumbBmp.close();
   return false;
 }