[gui] Please add spellcheck to the line editor

Like most people, I type fast and make a bunch of errors along the way. I really could use a spellchecker in the chat line editor.

The issue names only the GUI chat line editor and does not identify files, tests, platforms, or a spellchecking library. Start by locating the line editor entry point and checking how GUI input is implemented across the client. Done means the editor provides spellchecking while preserving normal chat entry, with appropriate coverage for the behavior.

Written by the indexing model from the issue text.