refactor(home): Address PR review feedback for Continue Reading cover

- Invert bookmark icon logic: show bookmark only when no cover image
  (as visual decoration), hide when cover is displayed to show more art
- Replace GfxRenderer::storeBwBuffer usage with HomeActivity's own
  buffer management (storeCoverBuffer/restoreCoverBuffer/freeCoverBuffer)
- Remove copyStoredBwBuffer() and freeStoredBwBuffer() from GfxRenderer
  as they enabled misuse of the grayscale buffer storage mechanism

lib/GfxRenderer/GfxRenderer.cpp

@@ -577,38 +577,6 @@ void GfxRenderer::restoreBwBuffer() {
   Serial.printf("[%lu] [GFX] Restored and freed BW buffer chunks\n", millis());
 }
 
-/**
- * Copy stored BW buffer to framebuffer without freeing the stored chunks.
- * Use this when you want to restore the buffer but keep it for later reuse.
- * Returns true if buffer was copied successfully.
- */
-bool GfxRenderer::copyStoredBwBuffer() {
-  // Check if all chunks are allocated
-  for (const auto& bwBufferChunk : bwBufferChunks) {
-    if (!bwBufferChunk) {
-      return false;
-    }
-  }
-
-  uint8_t* frameBuffer = einkDisplay.getFrameBuffer();
-  if (!frameBuffer) {
-    return false;
-  }
-
-  for (size_t i = 0; i < BW_BUFFER_NUM_CHUNKS; i++) {
-    const size_t offset = i * BW_BUFFER_CHUNK_SIZE;
-    memcpy(frameBuffer + offset, bwBufferChunks[i], BW_BUFFER_CHUNK_SIZE);
-  }
-
-  return true;
-}
-
-/**
- * Free the stored BW buffer chunks manually.
- * Use this when you no longer need the stored buffer.
- */
-void GfxRenderer::freeStoredBwBuffer() { freeBwBufferChunks(); }
-
 /**
  * Cleanup grayscale buffers using the current frame buffer.
  * Use this when BW buffer was re-rendered instead of stored/restored.

lib/GfxRenderer/GfxRenderer.h

@@ -90,10 +90,8 @@ class GfxRenderer {
   void copyGrayscaleLsbBuffers() const;
   void copyGrayscaleMsbBuffers() const;
   void displayGrayBuffer() const;
-  bool storeBwBuffer();       // Returns true if buffer was stored successfully
-  void restoreBwBuffer();     // Restore and free the stored buffer
-  bool copyStoredBwBuffer();  // Copy stored buffer to framebuffer without freeing
-  void freeStoredBwBuffer();  // Free the stored buffer manually
+  bool storeBwBuffer();    // Returns true if buffer was stored successfully
+  void restoreBwBuffer();  // Restore and free the stored buffer
   void cleanupGrayscaleWithFrameBuffer() const;
 
   // Low level functions