feat: add BRL (R$) currency support

[smllb]

feat: add Brazilian Real (R$) currency support

Summary

Adds detection, storage, and display of Brazilian Real (R$) prices. Previously
any non-mapped currency symbol fell through to the generic $ pattern and was
recorded as USD, which was wrong for stores like amazon.com.br.

What changed

Backend

• backend/src/utils/priceParser.ts

  • Add R$ / BRL to currencyMap and a dedicated regex pattern placed
    before the generic $ one (so R$ isn't swallowed by the dollar match).
  • The new pattern handles the Brazilian numeric format 1.234,56 (dot
    thousands separator, comma decimal) as well as the short form 284,90.
  • Include R$ in extractPricesFromText so it's considered when scanning
    raw HTML.

• backend/src/routes/products.ts

  • The confirm-price path previously hardcoded 'USD' when recording the
    initial price history entry. It now records the currency of the candidate
    the user actually selected (selectedCurrency), falling back to the
    detected price currency and finally 'USD'.

Frontend

• frontend/src/utils/formatPrice.ts (new)

  • Shared formatPrice() helper using Intl.NumberFormat, so any currency
    (including BRL) formats correctly and is locale-aware.

• PriceChart.tsx, ProductCard.tsx, PriceSelectionModal.tsx,
  ProductDetail.tsx

  • Replaced the hardcoded € / £ / CHF / $ symbol map with the shared
    formatter. BRL now renders as R$ 284,90 instead of $284.90.

• PriceSelectionModal.tsx, Dashboard.tsx, api/client.ts

  • Thread the selected candidate's currency through to the backend so the
    recorded currency matches what the user picked.

Why

The existing parser mapped a fixed set of symbols and fell back to $/USD
for everything else. That made tracked BRL prices display and store as USD.
This change is additive: all previously supported currencies behave exactly as
before, and BRL (plus any other Intl.NumberFormat-supported currency) now
works correctly.

Testing

• Verified parsePrice against R$284,90, R$5.256,36, R$1.299,00,
  R$499,90, and $29.99 (BRL cases parse to correct values + BRL,
  USD unaffected).
• tsc typecheck and production builds pass for both backend and
  frontend.
• End-to-end: adding an amazon.com.br product records history as BRL and
  the UI shows R$ formatting in the modal, cards, and price chart.