Support AI translation for question and answer

## Feature request

Add AI-assisted translation for question and answer drafts.

### Requirements

- Allow users to translate question titles and content before submission.
- Allow users to translate answer content before submission.
- Preserve Markdown, code blocks, URLs, HTML, mentions, and placeholders.
- Use the configured AI provider, model, and site language.
- Provide an administrator setting to enable or disable translation.
- Handle provider errors with actionable messages.
- Keep translation opt-in at the editor level so users can review the generated draft before replacing their content.