fix: fixed the available balance computation when stake is involved

[frol]

Currently, we did not hit the corner case since there are only a few accounts that do real stake.

See https://nomicon.io/Economics/README.html#state-stake

def check_storage_cost(account):
    # Compute requiredAmount given size of the account.
    requiredAmount = sizeOf(account) * storageAmountPerByte
    return Ok() if account.amount + account.locked >= requiredAmount else Error(requiredAmount)

and also the matching implementation in nearcore: https://github.com/near/nearcore/blob/a480d6eb3d9c6542100efdc6364eb89daac69383/core/runtime-configs/src/lib.rs#L75-L95

/cc @icerove

[chadoh]

Do I understand correctly that this means that the same token balance can be used both for storage staking and delegation/validator staking? That the same pot of tokens can do double-duty for storage & delegation?

[chadoh]

If this is true, we need to update this document https://docs.near.org/docs/concepts/storage-staking#how-does-nears-design-align-incentives

Storage-staked tokens are unavailable for other uses, such as validation staking. This increases the yield that validators will receive.

[frol]

That the same pot of tokens can do double-duty for storage & delegation?

@chadoh Correct. Indeed, the documentation needs to be updated.