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/GfxRenderer/GfxRenderer.h

@@ -68,6 +68,8 @@ class GfxRenderer {
   void drawImage(const uint8_t bitmap[], int x, int y, int width, int height) const;
   void drawBitmap(const Bitmap& bitmap, int x, int y, int maxWidth, int maxHeight, float cropX = 0,
                   float cropY = 0) const;
+  void drawBitmap1Bit(const Bitmap& bitmap, int x, int y, int maxWidth, int maxHeight) const;
+  void fillPolygon(const int* xPoints, const int* yPoints, int numPoints, bool state = true) const;
 
   // Text
   int getTextWidth(int fontId, const char* text, EpdFontFamily::Style style = EpdFontFamily::REGULAR) const;
@@ -97,8 +99,8 @@ class GfxRenderer {
   void copyGrayscaleLsbBuffers() const;
   void copyGrayscaleMsbBuffers() const;
   void displayGrayBuffer() const;
-  bool storeBwBuffer();  // Returns true if buffer was stored successfully
-  void restoreBwBuffer();
+  bool storeBwBuffer();    // Returns true if buffer was stored successfully
+  void restoreBwBuffer();  // Restore and free the stored buffer
   void cleanupGrayscaleWithFrameBuffer() const;
 
   // Low level functions