Large documents in persistant memory

After implementing #12 it was discovered that larger GUI documents could not be stored in the persistant flash memory. This is due to a known limitation:

String values are currently limited to 4000 bytes. This includes the null terminator. Blob values are limited to 508,000 bytes or 97.6% of the partition size - 4000 bytes, whichever is lower.

This means that larger GUI documents than 4000 bytes are not going to be able to be stored upon uploading remotely.
The workaround for this is to hard-code the large GUI when compiling a sketch, and passing it to mgui_init().

This limitation is not necessarily a big issue that needs to be solved, but it could be a good idea to implement a smart workaround for this so that the user gets a better experience using MicroGUI.

首先检查 issue 中描述的 ESP-IDF NVS 闪存大小限制，以及 mgui_init() 周围的文档加载路径。确定远程上传的 GUI 文档是如何存储的，并将完成标准定义为：允许存储和远程使用大于 4000 字节的文档，而不依赖硬编码的 sketch。