Your script is unique, I like the design. Unfortunately, your coinflip isn't truly provable fair. Let me explain how.
Since The house (xhowl) has access to the server seed, nonce and the client seed, xhowl can use this unfairly. Since these 3 variables will reveal the player roll number, Xhowl can create a dummy account and use this knowledge to their advantage. They can adjust their(dummy account's) client seed in such a way that it will result in a win for them as they have access to the server seed. So basically the players are trusting xhowl here that they won't screw them over. Hence why CoinFlip isn't fair.
Here's an example
server seed: abcdefghijk (The player doesn't knows this)
client seed (Me): zdxasdassa
nonce: 2
Theoretical Number rolled: 36
Now let's say I open a bet for 0.01 BTC and xhowl decides to screw me over. Since you guys have access to
server seed: abcdefghijk
client seed: 123wqdasd (You guys can test this out and choose a client seed for your dummy account in such a way that the rolled number will be higher than 36)
nonce: 6 (This again can be adjusted as you guys will have total access)
Number rolled: 50
Making an easy profit of 0.01 BTC.
