enhancement: build real-time fiat price converter for contributions

[devmocrea]

Description

Integrate a public price API to show the real-time USD value of CELO and STX contribution amounts directly inside the transaction modals.

Technical Specification

• Add a helper function in apps/web/src/lib/utils.ts to fetch current CELO/USD and STX/USD prices from the CoinGecko API.
• Render the calculated USD value beneath the contribution input fields in real time.

Acceptance Criteria

• Dynamic conversion estimates display accurately, updating periodically on input changes.