i18n: extract strings — onboarding, about & modals

[Davidnet]

Part of the frontend internationalization effort (see Epic). Depends on the infra issue.

Files

• src/frontend/src/components/onboarding/WelcomeModal.tsx (move string-bearing data structures, e.g. card title/desc arrays, into keys)
• src/frontend/src/components/about/AboutView.tsx
• src/frontend/src/components/modals/CACertSetupModal.tsx
• src/frontend/src/components/modals/MisclassificationModal.tsx