fix: Show warning if freq is low or not set

[mrv777]

Description

• Show warning if frequency is null or low as mentioned here: Onboarding pain: empty frequency value bitaxe#203

[alltheseas]

@mrv777 I see you added a null/low frequency warning - thank you.

One thing to consider from the perspective of a non-technical Bitaxe newbie - the newbie does not know what a low value is. Consider adding some context (for instance, as a tooltip to frequency field)- e.g. "Range of X - Y is typical value. Values below Z are considered low. "

I can create a new issue for this if helpful.