Add cover image display in *Continue Reading* card with framebuffer caching (#200)

## Summary

* **What is the goal of this PR?** (e.g., Fixes a bug in the user
authentication module,

Display the book cover image in the **"Continue Reading"** card on the
home screen, with fast navigation using framebuffer caching.

* **What changes are included?**

- Display book cover image in the "Continue Reading" card on home screen
- Load cover from cached BMP (same as sleep screen cover)
- Add framebuffer store/restore functions (`copyStoredBwBuffer`,
`freeStoredBwBuffer`) for fast navigation after initial render
- Fix `drawBitmap` scaling bug: apply scale to offset only, not to base
coordinates
- Add white text boxes behind title/author/continue reading label for
readability on cover
- Support both EPUB and XTC file cover images
- Increase HomeActivity task stack size from 2048 to 4096 for cover
image rendering

## Additional Context

* Add any other information that might be helpful for the reviewer
(e.g., performance implications, potential risks, specific areas to
focus on).

- Performance: First render loads cover from SD card (~800ms),
subsequent navigation uses cached framebuffer (~instant)
- Memory: Framebuffer cache uses ~48KB (6 chunks × 8KB) while on home
screen, freed on exit
- Fallback: If cover image is not available, falls back to standard
text-only display
- The `drawBitmap` fix corrects a bug where screenY = (y + offset) scale
was incorrectly scaling the base coordinates. Now correctly uses screenY
= y + (offset scale)

src/activities/home/HomeActivity.h

@@ -14,8 +14,13 @@ class HomeActivity final : public Activity {
   bool updateRequired = false;
   bool hasContinueReading = false;
   bool hasOpdsUrl = false;
+  bool hasCoverImage = false;
+  bool coverRendered = false;      // Track if cover has been rendered once
+  bool coverBufferStored = false;  // Track if cover buffer is stored
+  uint8_t* coverBuffer = nullptr;  // HomeActivity's own buffer for cover image
   std::string lastBookTitle;
   std::string lastBookAuthor;
+  std::string coverBmpPath;
   const std::function<void()> onContinueReading;
   const std::function<void()> onReaderOpen;
   const std::function<void()> onSettingsOpen;
@@ -24,8 +29,11 @@ class HomeActivity final : public Activity {
 
   static void taskTrampoline(void* param);
   [[noreturn]] void displayTaskLoop();
-  void render() const;
+  void render();
   int getMenuItemCount() const;
+  bool storeCoverBuffer();    // Store frame buffer for cover image
+  bool restoreCoverBuffer();  // Restore frame buffer from stored cover
+  void freeCoverBuffer();     // Free the stored cover buffer
 
  public:
   explicit HomeActivity(GfxRenderer& renderer, MappedInputManager& mappedInput,