Add gauges for CAN EGT channels 3–6 and a max EGT gauge

We recently added support for CAN-based EGT devices, for example the Ecumaster 6-channel EGT module.

At the moment, the dashboard/gauges only expose EGT channel 1 and EGT channel 2. We are missing individual gauges for channels 3, 4, 5, and 6.

It would also be very useful to add a separate maximum EGT gauge , showing the highest temperature value from all available EGT channels. A max EGT gauge would provide a quick safety/diagnostic value without needing to watch all channels individually.