feat(home): Add cover image thumbnail to Continue Reading card

Display book cover image as background in the Continue Reading card on
the home screen, improving visual identification of the current book.

Key changes:
- Add thumbnail generation (thumb.bmp) for EPUB and XTC/XTCH files
  - Uses same dithering/scaling algorithms as sleep screen covers
  - Target size: 240x400 (half screen) for optimal Continue Reading card fit
- Add JpegToBmpConverter::jpegFileToBmpStreamWithSize() for custom target sizes
- Add GfxRenderer::copyStoredBwBuffer() and freeStoredBwBuffer() for
  framebuffer caching to maintain fast navigation performance
- Add UTF-8 safe string truncation for Korean/CJK text in title/author display
- Draw white boxes behind title/author text for readability over cover image
- Increase HomeActivityTask stack size to 4096 for cover image rendering
- Add bounds checking in XTC thumbnail generation to prevent buffer overflow

src/activities/home/HomeActivity.h

@@ -13,8 +13,12 @@ class HomeActivity final : public Activity {
   int selectorIndex = 0;
   bool updateRequired = false;
   bool hasContinueReading = false;
+  bool hasCoverImage = false;
+  bool coverRendered = false;      // Track if cover has been rendered once
+  bool coverBufferStored = false;  // Track if cover buffer is stored
   std::string lastBookTitle;
   std::string lastBookAuthor;
+  std::string coverBmpPath;
   const std::function<void()> onContinueReading;
   const std::function<void()> onReaderOpen;
   const std::function<void()> onSettingsOpen;
@@ -22,7 +26,7 @@ class HomeActivity final : public Activity {
 
   static void taskTrampoline(void* param);
   [[noreturn]] void displayTaskLoop();
-  void render() const;
+  void render();
   int getMenuItemCount() const;
 
  public: