Refactor: Remove integrations and add Modbus TCP

[google-labs-jules]

This PR implements the requested changes to the SmartEVSE firmware:

1. Removed MQTT integration (code, config, UI).
2. Removed REST API integration (removed endpoints for external control like /currents, /ev_meter, /ev_state, while maintaining /settings for the Web UI).
3. Removed Home Battery integration.
4. Integrated Modbus TCP Server on port 502, reusing the existing Modbus RTU register mapping and logic, with mutex protection for thread safety.
5. Verified OCPP 1.6j compliance settings in initialization.

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PR created automatically by Jules for task 4620606358722400313 started by @webdotpulse

[google-labs-jules]

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