feat(i18n): implement comprehensive internationalization (i18n)

[0dillon]

Summary

As HaloPay expands into LATAM and Francophone Africa, English-only interfaces are a hard blocker for adoption. The codebase currently has dozens of hardcoded English strings scattered across React components, making localization impossible.

We need to architect a robust internationalization layer using next-intl. This requires extracting every user-facing string into structured JSON dictionaries. The architecture must support dynamic locale routing, allow the merchant to switch languages seamlessly via the settings modal, and automatically detect the device's native language preference on first boot.

Acceptance Criteria

• Integrate next-intl and configure locale routing.
• Extract all hardcoded strings into en.json, es.json, and fr.json.
• Build a language selector dropdown in the Merchant Settings modal.
• Ensure formatting for currencies and dates respects the active locale.

Tech Stack

Next.js 14, next-intl, React.