Feature request: Warn against invalid `hex` input

**Is your feature request related to a problem? Please describe.**

I forgot to switch the XOR key input field from hex to UTF8. Ideally, CyberChef would have warned if the input contains non-whitespace, non-[0123456789abcdef] inputs.

https://cyberchef.org/#recipe=XOR(%7B'option':'Hex','string':'Why%20is%20this%20allowed'%7D,'Standard',false)&input=U29tZSB0ZXN0IHN0cmluZw

**Describe the solution you'd like**

A warning or something would be nice to see

**Additional context**

Wasted like 2h in flare-on because of this 🙃

Start by locating the XOR operation and the handling for its Hex key input, then reproduce the behavior with the recipe and input linked in the issue. Check how operation inputs surface validation feedback. Done means invalid non-whitespace characters in Hex input produce a visible warning, while hexadecimal characters and whitespace remain accepted.

Written by the indexing model from the issue text.