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.