sponsor withdrawal flow

[Truphile]

closes
issue #12

I have completed the implementation of the sponsor withdrawal functionality.

Key Changes:

• src/services/sponsors.service.ts: Created a new service to handle fetching
  pool information, initiating withdrawals (obtaining unsigned XDR), and
  submitting signed transactions to the API.
• src/hooks/useTransaction.ts: Implemented a reusable hook for the Stellar
  transaction flow. It handles signing with the Freighter extension and
  submitting the signed XDR, with built-in loading and error state
  management.
• src/pages/Sponsors.tsx: Transformed the placeholder page into a functional
  Sponsor Dashboard.
  • Added a Pool Overview section showing APY, total deposits, and
    liquidity details.
    Verification:
• Ran npm run build successfully, ensuring no TypeScript or build errors.
• Verified that no any types are used in the new or modified code.
• Confirmed the flow matches the requirements: Preview -> POST /withdraw ->
  Sign -> Submit -> Success Display.

This implementation resolves the issue where sponsors were unable to withdraw
their funds.