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.

まず、issue に記載されている ESP-IDF NVS フラッシュのサイズ制限と、mgui_init() 周辺のドキュメント読み込み経路を確認します。リモートからアップロードされた GUI ドキュメントがどのように保存されるかを特定し、ハードコードされたスケッチに依存せずに 4000 バイトを超えるドキュメントを保存してリモートで使用できることを完了条件として定義します。