* feat: support outbound proxy for Chromium sessions
Routes WhatsApp Web traffic through an HTTP/HTTPS/SOCKS5 proxy when
PROXY_URL is set. Optional PROXY_API_KEY enables auth by passing the
key as the basic-auth username (the format API-key proxies expect).
No behavior change when unset.
Useful for deployments behind egress filters or where outbound
WhatsApp traffic must originate from a specific IP.
* docs: 📝 rewrite proxy auth part
* fix: 🐛 move api key value to passwd
* fix: 💄 change var naming
* fix: 🎨 change var ref in docs and data passing
