Implement SAC Transfer Functionality

[ShantelPeters]

This PR implements the functionality to enable interactions with Stellar Asset Contracts (SACs) in the Basic Payment App.

Changes Made

• Implemented createContractTransferTransaction function in src/lib/stellar/transactions.js
• Function handles both regular payments and SAC transfers
• Detects contract addresses using StrKey.isValidContract()
• Uses Contract.call() for SAC transfers
• Maintains existing functionality for regular payments

Addressing Previous Comments

• Moved the SAC transfer logic from walletStore.sign() to a separate function in transactions.js
• Transaction is now built before being presented to the user for signing
• The function checks whether the recipient address is a contract or a regular address
• Consistent with other transaction-creating functions in the module

TODO

• Update previewPaymentTransaction in +page.svelte to use the new createContractTransferTransaction function
• Implement RPC server simulation for contract interactions (if required)

Testing

• Tested with both regular Stellar accounts and contract addresses
• Verified correct handling of different asset types

Please review the changes and let me know if any further modifications are needed.