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/RecentBooksStore.cpp

@@ -38,6 +38,15 @@ void RecentBooksStore::addBook(const std::string& path, const std::string& title
   saveToFile();
 }
 
+void RecentBooksStore::removeBook(const std::string& path) {
+  auto it =
+      std::find_if(recentBooks.begin(), recentBooks.end(), [&](const RecentBook& book) { return book.path == path; });
+  if (it != recentBooks.end()) {
+    recentBooks.erase(it);
+    saveToFile();
+  }
+}
+
 void RecentBooksStore::updateBook(const std::string& path, const std::string& title, const std::string& author,
                                   const std::string& coverBmpPath) {
   auto it =