Async processing of thumbnails

src/activities/home/HomeActivity.h

@@ -1,4 +1,5 @@
 #pragma once
+#include <cstddef>
 #include <functional>
 #include <vector>
 
@@ -18,6 +19,7 @@ class HomeActivity final : public Activity {
   bool hasOpdsUrl = false;
   bool coverRendered = false;      // Track if cover has been rendered once
   bool coverBufferStored = false;  // Track if cover buffer is stored
+  size_t nextRecentCoverIndex = 0;
   uint8_t* coverBuffer = nullptr;  // HomeActivity's own buffer for cover image
   std::vector<RecentBook> recentBooks;
   void onSelectBook(const std::string& path);