feat(desktop): allow toggling spellchecker

**Motivation**

It gets very annoying very quickly to see red squiggles under all non-English text I write in the chat window.

**Proposed solution**

Implement a toggle in settings that disables spellcheck.

**Alternatives considered**

Finding a client with a better UI.

**Additional context**

Look at pretty much any other chat client.

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I searched existing issues and found nothing related.

Start by locating the desktop settings UI and the chat window's spellcheck behavior. Add a settings toggle that disables spellcheck for chat text, then verify that changing the setting removes the red squiggles without affecting other chat functionality.

Written by the indexing model from the issue text.