The latest firmware has added support for I2C LCD for the microcontroller-based OpenSprinkler. We haven’t added the same support for OSPi yet. But on Github there is a pull request that has implemented I2C LCD for OSPi and OSBo:
https://github.com/OpenSprinkler/OpenSprinkler-Firmware/pull/8
I haven’t had time to carefully check the pull request, but you can feel free to experiment with denisfrench’s code provided there.
You can host the web UI on RPi itself. We don’t include this in the preconfigured SD card because we make frequent changes to the UI and to get the latest changes you would have to update your local copy manually. So the default option is to use the UI assets hosted on opensprinkler server, this way updates are automatically accessible to everyone.