Add Next.js web frontend with friends, expenses, and multi-currency balances

[pdhruv93]

Summary

• Adds the full Next.js 15 web frontend (apps/web) with auth (login/register), home dashboard, friends page, profile management, and expense creation
• Implements per-currency balance display — each currency independently shows "owes you" (green) or "you owe" (red) instead of incorrectly summing across currencies
• Friends page accordion headers show per-currency balances alongside friend details, matching the home page summary
• All UI strings externalized via next-intl for i18n readiness
• Updates README with friends page, invited user flow, i18n support, and multi-currency balance details

Key changes

• 62 files changed across apps/web/ (new), README.md, docker-compose.yml, and pnpm-lock.yaml
• Pages: /login, /register, / (home), /friends, /profile
• Auth: API key stored in HTTP-only cookie, middleware-based route protection
• Balance bug fix: Balance direction is now computed per-currency, not by summing all currencies into one number
• Docker: Web frontend added to docker-compose.yml