feat(onboarding): add interface language selection

[zhongwater123]

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

Testing

• all test/components/onboarding*.test.js tests plus test/helpers/uiLanguageStartup.test.js (Node 24.19.0; 36 passed)
• npm run typecheck (Node 24.19.0)
• npm run i18n:check (Node 24.19.0)
• targeted ESLint and Prettier checks for the six hardening files
• npm run build:renderer (production renderer build)

The full Windows test run completed 2,577 tests (2,427 passed, 10 skipped), with 139 Windows/environment test failures remaining. They are dominated by Electron test mocks that leave app.isPackaged undefined in debugLogger.js; they are outside this scoped onboarding change and were not modified here.

Closes #1758