Channel keys need to tell what format they're expected to be entered in

[KyleMaas]

Brought this up here meshtastic/web#196 and here meshtastic/firmware#3855 as issues with the web side and the documentation, respectively. But the Android app has this issue as well.

There's nothing in the interface that tells you what format keys are expected to be in - hex, Base64, or even just freeform-and-hashed. As such, it took me some trial and error to generate and set a key, because my initial assumption based on the docs referring to the 0x01 primary channel was that it was expecting a string of hex digits, but it would just silently fail if I did that. There should be some kind of hint in the UI that tells that it's expecting a Base64-encoded string.