feat: sponsor withdrawal flow with transaction signing

[EmeditWeb]

Problem

The withdraw form on Sponsors page has no actual
transaction signing. Sponsors cannot withdraw their
funds.

What To Build

When sponsor submits withdrawal shares amount:

1. Show preview: shares -> USDC calculation
2. Call POST /liquidity/withdraw with shares
3. API returns unsigned XDR
4. Pass XDR to Freighter signTransaction()
5. Submit signed XDR to Stellar
6. Show success with amount returned and profit

Files To Touch

• src/pages/Sponsors.tsx
• src/services/sponsors.service.ts
• src/hooks/useTransaction.ts

Acceptance Criteria

• Preview shows correct USDC calculation
• Full withdrawal flow works end to end
• Transaction hash shown on success
• Error shown if user rejects in wallet

Mandatory Checks Before PR

• npm run build passes
• No any types
• PR references this issue

[Truphile]

Hi @[MaintainerUsername], I'll take this on.

Here is the exact implementation checklist I will follow to wire up the complete end-to-end Freighter signing flow for sponsor withdrawals:

⚙️ Technical Checklist

• [sponsors.service.ts] API Integration: Implement the wrapper for POST /liquidity/withdraw to submit the calculated shares payload and safely handle the unsigned transaction XDR string returned from the backend.
• [useTransaction.ts] Wallet Orchestration: * Pipe the unsigned XDR directly to Freighter's signTransaction().
  • Implement an explicit catch block for wallet rejections to satisfy the error feedback acceptance criteria.
  • Execute the on-chain submission to Stellar once the user authorizes the payload.
• [Sponsors.tsx] UI/UX & Live States:
  • Preview Calculator: Build the reactive shares-to-USDC conversion view before submission.
  • State UI: Handle loading, wallet-pending, error-catch, and success screens. On success, pull the transaction hash, total returned USDC, and computed profit directly from the receipt.

I'll ensure all components are strictly typed with zero any variables and that npm run build compiles completely clean. Please assign this over so I can start on the hook integration!

[grantfox-oss]

🦊 GrantFox — @Truphile has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #12)
2. Your PR will be reviewed by the StepFi-app maintainers

Good luck! Track your progress on GrantFox.