chore: add .env.example for backend and frontend

[however-yir]

Summary

Add example environment templates for the backend and frontend.

Why

This project requires both backend service configuration and frontend runtime settings, but new users currently have to read existing files and infer what to override first. Small templates make the setup easier without changing runtime behavior.

Scope

• add a root .env.example for backend-related settings
• add front-end/.env.example for the frontend Vite config
• add a short README note that points readers to both templates

Testing

• template-only changes
• reviewed the variable names against the current backend and frontend configuration files