fix: Lag before displaying covers on home screen (#721)

## Summary

Reduce/fix the lag on the home screen before recent book covers are
rendered

## Additional Context

We were previously rendering the screen in two steps, delaying the
recent book covers render to avoid a lag before the screen loads.
In this PR, we are now doing that only if at least one book doesn't have
the cover thumbnail generated yet. If all thumbs are already generated,
we load and display them right away, with no lag.

---

### AI Usage

While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it
helps set the right context for reviewers.

Did you use AI tools to help write this code? _**NO **_

src/activities/home/HomeActivity.h

@@ -17,10 +17,8 @@ class HomeActivity final : public Activity {
   SemaphoreHandle_t renderingMutex = nullptr;
   int selectorIndex = 0;
   bool updateRequired = false;
-  bool hasContinueReading = false;
   bool recentsLoading = false;
   bool recentsLoaded = false;
-  bool recentsDisplayed = false;
   bool firstRenderDone = false;
   bool hasOpdsUrl = false;
   bool coverRendered = false;      // Track if cover has been rendered once
@@ -41,7 +39,8 @@ class HomeActivity final : public Activity {
   bool storeCoverBuffer();    // Store frame buffer for cover image
   bool restoreCoverBuffer();  // Restore frame buffer from stored cover
   void freeCoverBuffer();     // Free the stored cover buffer
-  void loadRecentBooks(int maxBooks, int coverHeight);
+  void loadRecentBooks(int maxBooks);
+  void loadRecentCovers(int coverHeight);
 
  public:
   explicit HomeActivity(GfxRenderer& renderer, MappedInputManager& mappedInput,