SVGHMI: added a watchdog. To ensure that the whole chain is checked, watchdog use a periodic echo of a hearteat variable. JS client code systematically register /HEARTBEAT at 1s update freq, and reacts on updates of /HEARTBEAT by systematically incrementing it. C code catch /HEARTBEAT update and feeds python-implemented watchdog. For now, watchdog does nothing when tiggered

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<ModbusRequest xmlns:xsd="http://www.w3.org/2001/XMLSchema" Function="04 - Read Input Registers" SlaveID="0" Start_Address="0"/>