Add handling for BTC and ETH collateral withdrawals

[parrot9design]

When there's not enough ETH / BTC to withdraw collateral to the chain it was deposited from, we should provide people with the option to withdraw to a different blockchain.

Here's how that would look:

• Click Withdraw {amount} {asset} to refresh the modal with the updated values.
• If the amount on that chain is 0, the red text should say 0 {asset} available on {chain} and we can omit the second line.

• Click Blockchain to open a dropdown that shows which chains you can withdraw to, and how much is available there:

• Choose a blockchain – if you haven't connected a wallet, trigger the blockchain-specific connect wallet prompt.

• Then update the modal to include the active blockchain, address, and the cross-chain fee. The button label should also be updated to say Withdraw to {chain}: