WalletSnapshot.canSpend() calculation limitation

[HonzaR]

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

Once we know how to solve this properly, we'd like to address this inlined comment:

// Note this check is not entirely correct - it does not calculate the resulting fee using the new Proposal API. It's
// fine for now, but it's subject to improvement later once we figure out how to handle it in such cases.
fun WalletSnapshot.canSpend(amount: Zatoshi): Boolean = spendableBalance() >= amount

Describe the solution you'd like

For example, can we count the latest calculated and stored fee value with a fallback to 10K Zatoshi as a default value?

Alternatives you've considered

Additional context