Add ESP8266, ESP32-C6, ESP32-C2, ESP32-H2 to chip selector with
optgroup layout. Generalize UI text from ESP32-S3-specific to
board-agnostic. Add esptool merge_bin examples for ESP32, ESP32-S3,
and ESP8266.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>