Fix: add utility to retrieve token balance on Stellar

[GuiltyCrownne]

Summary:

• Adds a reusable utility to fetch a user’s Stellar token balance and accompanying tests.
• Changes
• Add src/utils/getTokenBalance.ts using stellar-sdk (Horizon testnet by default).
• Export server for easier mocking in tests.
• Add src/utils/getTokenBalance.test.ts with 4 cases (custom token, no holding, XLM, error).
• Add stellar-sdk dependency.

Tests

• Jest tests cover success, no balance, native asset, and error fallback to '0'.
• Notes
• Safe defaults; returns '0' on errors or missing assets.

Closes #242

[Marchand-Nicolas]

Thanks for your PR! I cannot test but it looks good!
Please make the requested change and test it!

[Marchand-Nicolas]

If you import new modules, please update the package.json accordingly.
Also, I installed the package manually, and it says "Module '"stellar-sdk"' has no exported member 'Server'. Did you mean to use 'import Server from "stellar-sdk"' instead?".
(Maybe because you have a different version of the package than me, so that's why it's important to update the package.json)