Update and align sample apps

[raleng]

Description

This PR aligns the general dev setup across sample apps.

Env vars

• Template file is called env-template.
• Local env file is .env.

Port handling

• Sample apps use increasing ports starting from 7310 to minimize conflicts.
• NextJS apps use a port check script to avoid NextJS auto-incrementing the port which would break the ngrok tunnel.
• Express apps use an error listener on EADDRINUSE.

App start script and ngrok tunnel

• npm run dev starts the app with an ngrok tunnel.
• npm run dev:app only starts the app.