## Summary
Adds `docs/adding-a-currency.md`, mirroring
`docs/adding-a-banking-provider.md` but for currencies. Distilled from
the NGN (#642) and GHS (#644) additions.
The gist: adding a currency is a config change, not a feature.
- **Required**: one entry in `config/currencies.php`. Validation (Form
Requests), the Inertia dropdown props, and conversion all auto-derive
from it via `CurrencyOptions` / `CurrencyConversionService`.
- **Translation**: add the name to `lang/es.json` (enforced),
`lang/fr.json` optional. Flagged that `LocalizationTest` won't
auto-catch a missing one, since the name is translated in PHP, not
scanned from TS source.
- **Optional**: a custom short symbol in
`resources/js/utils/currency.ts`.
- **Two gotchas**: use the current ISO 4217 code (the GHC→GHS lesson
from #644), and verify the provider covers the code at the right rate
via the CDN URL.
## Testing
Docs-only change; no code touched.
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