feat: separate UI and model response languages

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Summary

Closes #757

This PR separates the Web interface locale from the default language used for model-generated responses.

Changes

• Add account-level response_language alongside language.
• Keep legacy configurations compatible by inheriting the old language when response_language is absent.
• Add independent controls under Settings > Appearance.
• Sync both preferences from the account settings API to browser storage.
• Make chat, question generation, co-writer, mastery path, quiz judge, and related capability defaults use response_language.
• Add backend, multi-user, and frontend regression tests.

Verification

• Backend regression suite: 102 passed.
• Frontend tests: 157 passed.
• TypeScript, ESLint, i18n parity, Next.js build, Ruff, and git diff --check passed.
• Manually verified in the running Docker service that UI language and model response language can be changed independently.

The existing wire_api work in the local checkout is intentionally excluded from this PR.