feat(onboarding): add interface language selection - #1803

Summary

• add a dedicated interface-language step to both account and guest onboarding routes
• apply the selected locale immediately through the existing setUiLanguage persistence path, without changing spoken/transcription languages
• use native radio controls for built-in keyboard and assistive-technology behavior
• keep supported UI languages and display metadata in one type-checked configuration source
• share the UI-language options with Settings and add copy for all 10 supported locales
• cover route/progress behavior, accessible single-selection rendering, and UI-language persistence isolation

Maintenance update

• synchronized the branch with main@81f8c71ed2d1abf9dd90dafcd9ae4a6da8432b48, including the new required-models gate, Screen Context permission, and compact onboarding refinements
• verified the merged account order remains auth → optional required-models → permissions → interface-language → transcription languages
• updated the upstream required-models progress integration assertion so the added interface-language step is counted explicitly

Testing

Run on Windows with Node.js 24.19.0:

• all 8 test/components/onboarding*.test.js files plus test/helpers/uiLanguageStartup.test.js — 45 passed
• ESLint for all JavaScript/TypeScript files in the PR — passed
• Prettier check for all 20 PR files — passed
• cd src && tsc --noEmit — passed
• node scripts/check-i18n.js — locale keys and placeholders are consistent
• cd src && vite build — passed

The earlier full Windows suite baseline was not rerun during this maintenance update. Its unrelated Electron mock failures remain outside this scoped onboarding change; the synchronized focused suite and production checks above are current.

Closes #1758