Autocorrect in the Chrome Extension

We have implemented hotkeys for opening a suggestion, and for actually applying the most likely suggestion without review. I'd love to extend this functionality to allow people to configure Harper to simply and directly apply the suggestion automatically, but only if it is high-confidence.

In the beginning, we can apply it to only spellchecking, then expand from there. I'm not sure how we'd feed confidence scores to the frontend. Perhaps `priority` is a good enough approximation?

Start by tracing the existing hotkey flows for opening and applying suggestions, then inspect how the frontend receives spellchecking suggestions and their priority. Define the high-confidence threshold and verify that only eligible spelling suggestions are applied automatically, while preserving the existing manual actions.

Written by the indexing model from the issue text.