Iteration 6: add lightweight multilingual UI copy

[HimanM]

Summary

• add a tiny frontend UI copy map for English, Sinhala, and Tamil with English fallback
• add a simple header language toggle and localize the main hero, quick actions, and input placeholder
• keep the shopping logic unchanged and document the verification in the feature checklist

Verification

• node --test --experimental-strip-types frontend/src/lib/ui-copy.spec.ts
• npm run lint
• npm run build
• Playwright fallback against http://localhost:3000 to verify English default, Sinhala toggle behavior, and English fallback restoration