Desired: Terrain following toggle

It would be helpful to have a dropdown on/off selector for the TERRAIN_FOLLOW parameter on the Data/Actions screen. Although in general it may be good to keep it enabled for safety at low to moderate altitudes, RTL, etc, it is better for endurance to simply fly level for most of the flight.

For commercial purposes where we have blocked off the config pages it would be nice to have this exposed to set on the fly, as I don't see any real downside concerns, as long as the tiles are loaded anyway (and maybe disabled if TERRAIN_ENABLE is 0?). It could be an example plugin that could be disabled for those who don't want it, which may be ideal.

The issue names the Data/Actions screen and the TERRAIN_FOLLOW and TERRAIN_ENABLE parameters; start by locating that screen and existing parameter controls. Clarify whether this belongs in the core UI or an optional plugin, including behavior when TERRAIN_ENABLE is 0, then verify the selector changes TERRAIN_FOLLOW safely.

Written by the indexing model from the issue text.