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)

lib/Epub/Epub.h

@@ -46,6 +46,8 @@ class Epub {
   const std::string& getAuthor() const;
   std::string getCoverBmpPath(bool cropped = false) const;
   bool generateCoverBmp(bool cropped = false) const;
+  std::string getThumbBmpPath() const;
+  bool generateThumbBmp() const;
   uint8_t* readItemContentsToBytes(const std::string& itemHref, size_t* size = nullptr,
                                    bool trailingNullByte = false) const;
   bool readItemContentsToStream(const std::string& itemHref, Print& out, size_t chunkSize) const;