docs(frontend): add env example template

[huazhuang80-star]

Summary

• add a tracked �pp/frontend/.env.example template for browser-safe NEXT_PUBLIC_* configuration
• document local defaults for API URL, Stellar network/RPC values, mock mode, role, and demo checklist flags
• allow the frontend template through the app-level .gitignore while keeping real .env* files ignored

Part of #98. The current main branch already has env examples for backend, mobile, onchain, and AI service; this fills the missing frontend template.

Testing

• git diff --check
• Not run: frontend dependency checks because
  ode_modules is not installed in this checkout