docs: clarify Windows quick start terminals

[WilliamK112]

Summary

• Clarify the README quick start for Windows PowerShell.
• Split dependency install, backend API, and frontend dev server into separate steps.
• Call out that npm run server and npm run dev should run in separate terminal windows.

Closes #3

Validation

• npm install
• npm run lint
• npm run build
• npm test
• Windows quick-start smoke: started backend and frontend in separate hidden PowerShell-launched processes, then confirmed http://127.0.0.1:3001/health and http://127.0.0.1:3010/ both returned HTTP 200.