Document env config and local dev setup

[sugan0tech]

Summary
Improve documentation for environment configuration and local development.

Context
The app will read API base URL from Vite env. Clear documentation helps new contributors set up quickly.

Tasks

• Add .env.example documenting VITE_API_BASE_URL and any other required variables.
• Update README with:
  • Dev commands: npm run dev, npm run build, npm run preview.
  • Lint/type-check commands and expected Node version.
  • Common troubleshooting (CORS, cookies on localhost, secure/sameSite behavior).
• Note deployment guidance for setting VITE_API_BASE_URL in CI/CD.

Acceptance Criteria

• New contributors can run the app locally following README without guesswork.
• .env.example is accurate and minimal.