Add cNGN token support to frontend token selector

[Ali6nXI]

Overview

The frontend currently defaults to USDC in the token selector dropdown.
We need to add cNGN (Nigerian Naira stablecoin on Stellar) as a
supported token — this is critical for local Nigerian users who want
to send remittances and gig salaries in their native currency without
converting to USD first.

Background

cNGN is Nigeria's regulated stablecoin pegged 1:1 to the Naira,
issued on Stellar. Adding it makes Stellar Drips the first streaming
payment tool natively supporting NGN-denominated drips — a huge
differentiator for the Lagos/Nigeria market.

Tasks

• Find the official cNGN asset address on Stellar testnet and mainnet
  Issuer: https://www.cngn.co
• Add a SUPPORTED_TOKENS constant in
  frontend/src/services/stellar.ts:

      export const SUPPORTED_TOKENS = {
        USDC: {
          symbol: 'USDC',
          name: 'USD Coin',
          address: 'USDC_CONTRACT_ADDRESS',
          decimals: 7,
          flag: '🇺🇸'
        },
        cNGN: {
          symbol: 'cNGN',
          name: 'Nigerian Naira',
          address: 'CNGN_CONTRACT_ADDRESS',
          decimals: 7,
          flag: '🇳🇬'
        }
      }

• Update CreateStreamForm.tsx token dropdown to use
  SUPPORTED_TOKENS — show flag + symbol + name
• Add cNGN as the first option in the dropdown (Nigeria-first)
• Display the token symbol on StreamCard.tsx next to amounts
• Add a helper formatAmount(amount, token) that formats
  correctly per token decimals
• Test on Stellar testnet with both USDC and cNGN streams
• Open a PR with screenshots showing the updated token selector

Acceptance Criteria

• Dropdown shows both cNGN and USDC with flags
• cNGN appears first in the list
• StreamCard correctly displays token symbol
• Amount formatting handles 7 decimal places correctly

Resources

• cNGN official site: https://www.cngn.co
• Stellar asset lookup: https://stellar.expert
• Current token selector: frontend/src/components/CreateStreamForm.tsx
• Stellar SDK assets: https://developers.stellar.org/docs