fix(frontend): sign and submit loan repayment transactions

[MJ-RWA]

Summary

Fixes the broken loan repayment flow where repayment transactions were built but never signed or submitted to the Stellar network.

The repayment flow now follows the correct transaction lifecycle:

Build transaction → Sign with wallet → Submit to network → Confirm success

Changes

• Updated the repayment API contract to handle unsignedTxXdr and networkPassphrase returned by the backend.
• Refactored the repayment operation to sign transactions through the wallet before submission.
• Removed the placeholder transaction hash that incorrectly used the loan ID.
• Ensured repayment success states are triggered only after successful network submission.
• Prevented gamification XP and success messages from being granted for transactions that were never executed.
• Improved handling of signing, submission, and network errors.

Problem

Previously, the frontend treated /loans/:loanId/repay as a fully executed transaction endpoint even though it only returns an unsigned transaction XDR. This allowed the UI to report a successful repayment without any transaction being signed or submitted to the Stellar network.

Testing

• Verified the updated repayment flow follows the required build → sign → submit lifecycle.
• Ran the relevant project validation checks and addressed any issues introduced by the changes.

Related Issue

Closes #1603