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.h

@@ -30,6 +30,9 @@ class RecentBooksStore {
   void updateBook(const std::string& path, const std::string& title, const std::string& author,
                   const std::string& coverBmpPath);
 
+  // Remove a book from the recent list by path
+  void removeBook(const std::string& path);
+
   // Get the list of recent books (most recent first)
   const std::vector<RecentBook>& getBooks() const { return recentBooks; }