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>

src/activities/reader/EpubReaderActivity.cpp

@@ -745,6 +745,9 @@ void EpubReaderActivity::onReaderMenuConfirm(EpubReaderMenuActivity::MenuAction
         epub->setupCacheDir();
 
         saveProgress(backupSpine, backupPage, backupPageCount);
+
+        // 5. Remove from recent books so the home screen doesn't show a stale/placeholder cover
+        RECENT_BOOKS.removeBook(epub->getPath());
       }
       xSemaphoreGive(renderingMutex);
       // Defer go home to avoid race condition with display task