ATMEGA644 only has 64K flash memory, for the web interface (image/html/css), I think it has more than 64K in size, so where does the web files get stored? I don’t have flash card installed.
Also, I’d like to build the firmware myself, github has 2.1.2, where is the latest 2.3 source code?
If you view the HTML source the controller sends for the home page, you will notice it sources a script from https://ui.opensprinkler.com. That is where the UI is stored.