[blb4github]

Summary

Integrates the configurable extra GPIO implementation originally developed by IgorYbema in feature/configurable-extra-gpio (reference commit 1149ef1656f8df65740d53278920e1ae159d614b). Original design and implementation credit goes to IgorYbema.

• Configurable input, input-pullup, and output modes: ESP8266 GPIO 1/3/16; ESP32 GPIO 33–37, in addition to fixed relays 21/47.
• Settings persistence, MQTT control and retained GPIO state reporting, live web interface, HTTP endpoint, and JSON output.
• Guards for pins reserved by OpenTherm and ESP8266 listen-only operation.
• UI relay naming based on physical relay pin rather than visible GPIO count.

Original MQTT interface intentionally preserved: <base>/gpio/extra/N serves as both command and retained state topic, just as in the original implementation. Fixed relay command topics are unchanged. A potential separate gpio_state namespace is deliberately deferred to a separate, future PR after acceptance.

Validation

• GPIO implementation previously tested on ESP32-S3 HeishaMon Large in combined experimental firmware.
• The isolated single-commit branch passes both ESP8266 and ESP32 firmware compilation in GitHub Actions.
• Dedicated GPIO-only hardware regression testing (especially output behavior across reboot and retained MQTT messages) is still desirable before deployment.

Scope

No webserver TCP transport changes; those are a separate proposal.