feat: enhance EInk display handler with improved resource management and non-blocking SPI transactions

main/display/eink_display_handler.h

@@ -28,6 +28,7 @@ private:
   // LVGL display and input device handles
   lv_display_t* _lvgl_display = nullptr;
   lv_indev_t* _lvgl_touch_indev = nullptr;
+  lv_draw_buf_t* _lvgl_draw_buf = nullptr;
 
   // Framebuffer
   uint8_t* _framebuffer = nullptr;