Add i18n support for additional languages

[maxmini0214]

DonFlow currently supports Korean and English. Adding more languages would help reach a wider audience.

Currently supported: ko, en
Requested: ja (Japanese), zh (Chinese), es (Spanish)

How to contribute:

1. Copy src/i18n/en.ts
2. Translate the key-value pairs
3. Register in src/i18n/index.ts

Good first issue — no app logic changes needed, just translation work.

[maxmini0214]

Progress update:

• ✅ Currency symbol now locale-aware (₩ for ko, $ for en) — commit d4de72f
• ✅ Demo data fully localized — English users see US merchants (Trader Joe's, Costco, Uber, etc.) with USD amounts (,000 salary) — commit d015686
• ⬜ Additional languages beyond ko/en (future)