Add local JavaScript plugin functions

[RDawg29]

Summary

• add persisted JavaScript plugin definitions with a default calculator example
• expose enabled plugins to OpenAI through the legacy chat completions function-calling API
• execute requested plugin JavaScript locally and send the function result back for a final assistant response
• add a settings modal for enabling, editing, adding, and removing plugins

Verification

• corepack yarn build
• opened the Vite app locally and verified Settings -> JavaScript plugins renders the plugin editor

Closes #499