Write `CONTRIBUTING.md` for the frontend repo

[Marvell69]

New frontend contributors have no guide for setting up the development environment, running tests, or following code conventions. A CONTRIBUTING.md is essential for a healthy open-source project.

Requirements and context

• Cover: Node version requirement, env variable setup, local development commands, testing commands, code style (ESLint + Prettier), PR requirements (screenshots for visual changes, test coverage, conventional commits), review process
• Include Stellar-specific setup (testnet account, Freighter install)
• Document MSW mock usage for local development

Suggested execution

git checkout -b docs/contributing

• Create CONTRIBUTING.md
• Test all instructions on a fresh machine
• Link from README

Example commit message
docs: add CONTRIBUTING.md for frontend contributors