Add MYR (Malaysian Ringgit) to supported currencies

[marcmeee]

Closes #485

What

• Adds Malaysian Ringgit (MYR) to the supported currency list
• MYR is already available via the Frankfurter API but was missing from the app UI

How

• Extended currency-data.json with MYR metadata
• Ensured currency resolution and conversion flow works unchanged

Testing

• Tested locally
• Created a group with MYR as base currency
• Added expenses in multiple currencies
• Verified conversion rates are fetched correctly via Frankfurter API