Add unit tests for currency exchange rate utilities

[aruneshwisdm]

Summary

The currency exchange rate module in `apps/web/lib/currency/exchange-rates.ts` contains utility functions for multi-currency conversion that have no unit tests. The synchronous `convertToBase` function and the caching logic are particularly well-suited for testing.

Functions to test

• `convertToBase(amount, fromCurrency, baseCurrency, rates)` — synchronous currency conversion using a pre-fetched rates map
• `convertCurrency(amount, fromCurrency, toCurrency)` — async conversion that fetches rates (mock the fetch)
• `getExchangeRates()` — rate fetching with caching and fallback logic

Files

• Source: `apps/web/lib/currency/exchange-rates.ts`
• Test file to create: `apps/web/tests/lib/currency/exchange-rates.test.ts`

Acceptance criteria

• `convertToBase` tested with: same currency (returns original amount), different currencies, unknown currency (returns original), zero amount
• `convertCurrency` tested with mocked fetch: successful API response, failed API response (falls back to static rates)
• `getExchangeRates` tested: returns cached rates when fresh, fetches new rates when cache is expired
• Edge case: unknown currency codes return the original amount unchanged
• All tests pass with `pnpm vitest run`

Getting started

You'll need to mock `fetch` for the async functions. Look at how Vitest's `vi.fn()` and `vi.spyOn(globalThis, 'fetch')` work. The `convertToBase` function is synchronous and needs no mocking — start there.

[nightcityblade]

Hi, I'd like to work on this. I'll submit a PR shortly.