Can I use the extra thermostat cable as C cable for wifi?
Go ahead, hook that blue wire up to C and use it for your Ecobee

As the terminal strip on your furnace control board is typical in that it has room for 2 wires on each screw, you can simply land the blue wire on the furnace control board's C terminal alongside the white wire that's already there.

BTW: the brown cable you're asking about can simply be left alone -- it's the cable that connects your thermostat's yellow wire (Y terminal) and the furnace control board's C terminal to the compressor contactor in your air conditioner so the thermostat can turn it on and off when things get sweltering in the summertime.