feat(ai): support translating question and answer drafts

[hgaol]

What changed

• Add an AI translation endpoint for question and answer draft content.
• Add translation controls to question and answer editors.
• Preserve Markdown and other structured content through the translation prompt.
• Add an administrator setting to enable or disable translation.
• Default translation to enabled for existing installations.
• Expose translation availability through site information.
• Add localized English and Chinese strings.
• Add tests for parsing, provider errors, configuration defaults, and translation requests.

screenshot

Closes #1610