Initial OpenEmber firmware

[superbeetle1973]

Summary

• Stand up OpenEmber as a standalone ESP32 PlatformIO firmware repository.
• Add firmware source, web UI, recipe engine, recipe schema/examples, hardware/flashing/API docs, README, license, and GitHub Actions CI.
• Fix standalone repo readiness issues found during validation: maintained ESP Async WebServer/AsyncTCP package names, missing 16 MB partition table, valid ESP32 filesystem partition subtype, repo source link, and Apple Silicon uploadfs troubleshooting note.

Validation

• python3 tools/validate_recipe.py passed locally with jsonschema installed: 3/3 recipes valid.
• /tmp/openember-pio-venv/bin/pio run -e esp32dev -e esp32dev-16mb passed locally.
• CI also validates recipes, builds both firmware environments, and builds the LittleFS image on Ubuntu.