feat: support outbound proxy for Chromium sessions (#131)

* 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

.env.example

@@ -55,4 +55,13 @@ TRUST_PROXY=FALSE
 
 ## CORS ##
 # OPTIONAL, CORS ALLOWED ORIGINS (comma separated), DEFAULT ALLOWS ALL
-# ALLOWED_ORIGINS=http://localhost:3000,http://example.com
+# ALLOWED_ORIGINS=http://localhost:3000,http://example.com
+
+## Outbound Proxy ##
+# OPTIONAL, ROUTES CHROMIUM (WHATSAPP WEB) TRAFFIC THROUGH AN OUTBOUND PROXY.
+# ACCEPTED FORMATS: http://host:port, https://host:port, socks5://host:port
+# PROXY_URL=http://10.0.0.10:8118
+# OPTIONAL, PROXY BASIC-AUTH USERNAME
+# PROXY_USERNAME=
+# OPTIONAL, PROXY BASIC-AUTH PASSWORD
+# PROXY_PASSWORD=