feat: Add Currency Exchange API and React UI

[devin-ai-integration]

Summary

Implements a full-stack currency exchange feature:

Backend (Spring Boot)

• Database: Flyway migration V2 creates currency and currency_rate tables with seed data (6 currencies, 6 exchange rate pairs)
• GET /currency: Returns all currencies keyed by currency code with country name, currency code, and currency name
• GET /exchange-rate/{fromCur}/{toCur}: Returns the exchange rate between two currencies (404 if pair not found)
• Domain entities: Currency and CurrencyRate in io.spring.core.currency
• MyBatis mapper: CurrencyMapper with XML mappings for database queries
• Service layer: CurrencyService with business logic
• Security: Both endpoints are publicly accessible (no auth required)

Backend Tests (JUnit 5)

• CurrencyApiTest — 3 tests: list all currencies, get exchange rate, 404 for unknown pair
• CurrencyServiceTest — 3 tests: service returns currency map, exchange rate, null for unknown pair

Frontend (React/Next.js)

• Currency List: Table displaying all currencies with code, name, and country
• Exchange Rate Calculator: Dropdown selectors + button to calculate exchange rates between any two currencies
• Page: /currency route integrating both components

Frontend Tests (Jest + React Testing Library)

• CurrencyList.test.tsx — 4 tests: renders table, headers, empty state, correct row count
• ExchangeRateCalculator.test.tsx — 5 tests: form rendering, validation errors, successful rate display, API error handling

UI Screenshot

Migration Ordering

• V1: Core schema tables (users, articles, etc.)
• V2: Currency tables + seed data (currency, currency_rate)
• V3: Application seed data (sample users, articles, tags)
• Test profile uses spring.flyway.target=2 to skip V3 seed data

Review & Testing Checklist for Human

• Start the backend (./gradlew bootRun) and verify GET /currency returns all 6 currencies
• Verify GET /exchange-rate/USD/INR returns {"fromCurrencyCode":"USD","toCurrencyCode":"INR","exchangeRate":"80.08"}
• Start the frontend (cd frontend && NODE_OPTIONS=--openssl-legacy-provider npm run dev) and visit http://localhost:3000/currency
• Test the exchange rate calculator by selecting USD -> INR and clicking "Get Exchange Rate"

Notes

• gradle.properties was added with JVM args to fix Spotless/google-java-format compatibility with Java 17
• Frontend babel config is scoped to Jest only (babel.jest.config.js) so it doesn't interfere with Next.js
• Addressed Devin Review findings: cleared stale results on validation errors, fixed Flyway migration ordering

Link to Devin session: https://partner-workshops.devinenterprise.com/sessions/994d158e00714fad9f07f37b78f0297a

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[devin-ai-integration]

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