feat: implement backend integration for onramp and offramp forms

[greatKhalifa-code]

This PR introduces the necessary backend infrastructure and frontend modifications to support real-time order creation and tracking. It replaces static mock data with live API calls for exchange rates and persists transaction states through a newly
implemented API layer.

Key Changes:

• API Endpoints:
  • POST /api/onramp/create-order: Handles onramp order initialization and data persistence.
  • POST /api/offramp/create-order: Manages offramp order creation.
  • GET /api/onramp/order/[orderId]: Provides a backend source of truth for order tracking.
  • PATCH /api/onramp/order/[orderId]/status: Enables real-time status updates (e.g., payment received, minting, completed) to be synchronized with the backend.
• Live Rate Integration:
  • Updated useOfframpRate to fetch real-time prices for USDC and XLM via the /api/exchange-rate endpoint, ensuring users see accurate conversion values based on current market data.
• User Experience:
  • Implemented isSubmitting states across OnrampCalculator and OfframpCalculator to handle asynchronous API delays gracefully.
  • Enhanced the "Continue" buttons with loading spinners and contextual labels (e.g., "Creating Order...") to prevent duplicate submissions and improve feedback.
• Data Consistency:
  • Modified useOrderTracking and useOrderStatusUpdates hooks to bridge the gap between local state (localStorage) and the backend API, providing a more robust and scalable tracking mechanism.

Verification:

• Tested end-to-end onramp flow: Form Input → Backend Order Creation → Status Polling.
• Tested offramp rate refreshing against live API data.

closes #126

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