Deploy contract to Stellar testnet + add contract address to frontend

[Ali6nXI]

Overview

The core Soroban streaming contract is implemented but not yet deployed
to Stellar testnet. This issue covers deploying it and wiring up the
real contract address in the frontend.

Tasks

• Install Stellar CLI and configure testnet network
• Fund a testnet account via Friendbot
• Build the contract: cargo build --target wasm32-unknown-unknown --release
• Deploy using: stellar contract deploy --wasm ...
• Copy the returned contract address
• Replace the TODO placeholder in frontend/src/services/stellar.ts
• Test create_stream and claim via CLI before updating frontend
• Open a PR with the updated contract address + deployment notes

Resources

• Stellar testnet: https://horizon-testnet.stellar.org
• Friendbot: https://friendbot.stellar.org
• Soroban deploy docs: https://developers.stellar.org/docs/build/smart-contracts/getting-started/deploy-to-testnet