Have you searched existing Github issues to see if someone has already requested this feature?
Yes

Is your feature request related to a problem? Please describe.

I like the simple interface of heynote to locally manage drafts of longer messages or markdown documents.

So if I want to spellcheck a draft, I always have to paste it somewhere else first before I send it out.

Describe the solution you'd like

A simple, built-in spellcheck in heynote.

Heynote should run the text through a basic spellcheck tool - one could either use the features provided by the operating systems, or one of the basic dictionary-based open source spellcheck libraries like gnu-aspell or similiar.
It should underline unknown words with a red, squiggly line, following the UX standards used in many other text editing applications.
Right clicking on an underlined word should provide the option to the a word to a custom dictionary of known words.

Next to the programming language choice, there is a spellcheck selection that allows you to select the language you're writing in, e.g. en-UK, de-De, fr-FR, etc.
It should be possible to disable the spellcheck, or choose different languages on a per-buffer-basis.
Language auto detection could be a stretch goal, but is not needed in the basic form, selecting a default in the config should be enough.

Describe alternatives you've considered
Please no AI/LLM integration or tools like grammarly. I'm using heynote explictly because its' simple and local-only, and doesn't send my data to 3rd party services. A simple local spellcheck would be enough.