types: `model` types shouldn't have signed values unless its meaningful - I think

[tcharding]

We overuse signed ints because it was found in the wild that Core has a tendency to return -1. However in model I don't think we should have signed ints unless its meaningful.

Found while reviewing #237 and also on master we have in model::wallet::GetTransaction:

    pub confirmations: i64, // Docs do not indicate what negative value means?

[0xZaddyy]

Looking at fields like change_position and confirmations these values are usually non-negative but can sometimes be -1 as a sentinel.
would it make sense to represent such fields in our model as Option?
this way:

• None can clearly express special cases like “no change” or “unconfirmed.”
• We avoid storing meaningless negative values in unsigned types.
  i just want to know if this approach aligns. Thank you!

[tcharding]

I rekon we should just convert to a u32 using crate::to_u32() and return the NumericErrror if its negative.