feat: Wire Convert form to live API and replace MOCK data

[AbuJulaybeeb]

• Added createSwap() to lib/api/transactions.ts to call POST /transactions/swap.
• Replaced MOCK_RATES with live exchange rate data fetched via GET /api/exchange-rates, adding a fallback error state for 500 errors or failures.
• Replaced MOCK_BALANCES with real data using the getBalances() function from lib/api/wallet.ts.
• Removed disabled={true} from the submit button and connected the form to createSwap(), handling loading, success, and error states gracefully.

Why it was done

The Convert form was completely non-functional, utilizing hardcoded mock data and an inactive submit button. This change activates the core conversion functionality by integrating with the live backend APIs, enabling users to actually swap currencies based on active rates.

How it was verified

• Form accurately populates balances from the backend using getBalances().
• Fetches rates dynamically and falls back gracefully, disabling the submit button and alerting the user when the backend is down (e.g. returns a 500 error).
• Successfully validates inputs (insufficient balance, zero amount) and calls POST /transactions/swap with the correct payload on submission.
• Handles success and error responses gracefully from createSwap().
  Closes [Wave 200pts] Feature: Wire Convert form to live API — replace MOCK_RATES/MOCK_BALANCES and enable the disabled submit button #170

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[portableDD]

@AbuJulaybeeb please resolve ci error, ci must be successful