docs: update README, env variables, and per-network WASM hash check

[NoobFaris]

Summary

This PR addresses out-of-date documentation and aligns environment variable handling with the multi-network features.

Closes #366

Changes

• Updated README.md tech stack versions (Next.js 16 / React 19), project structure, and corrected CLI commands from soroban to stellar.
• Updated .env.example to remove deprecated RPC/passphrase variables, added NEXT_PUBLIC_MERCURY_AUTH_TOKEN, and transitioned to network-specific token WASM hash variables.
• Updated useDeployToken hook to dynamically select WASM hashes based on active network state with clear validation errors.
• Created scripts/check-env.js to ensure all NEXT_PUBLIC_* references in source code exist in .env.example.

Type

• 📖 Documentation
• 🛠️ Refactor
• 🧪 Tests

Checklist

• My code follows the project's style guidelines
• I've updated documentation if needed
• I've tested on Stellar Testnet (if applicable)

[drips-wave]

@NoobFaris Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits