Optimistic stablesats balance check can block legitimate sends

[vindard]

Describe the bug
When paying a lightning invoice using USD (stablesats) wallet, we currently use the display-price-converted amount against the sats in the invoice to check whether there is enough balance to send before deciding if to execute a backend call. This can often be wrong for 2 reasons:

• the rates stablesats use internally come from derivatives exchanges and can vary significantly enough from spot price to wrongly block a send
• for intraledger stablesats-to-stablesats transactions the sats-denominated invoice is irrelevant since we discard that quote and settle the transaction in usd-only amounts internally

Expected behavior
The send attempt should be sent through to the backend and any balance checks will be executed there and communicated back up to the up via graphql error messages/codes