Improve network detection when connecting and signing a transaction

[sdfcharles]

Right now, if a testnet application asks to connect, Freighter will connect even if Mainnet is selected on Freighter. Then when the application asks to sign for a transaction, it will show the message below because of the network mismatch.
A better experience would involve:

• When connecting to a different network, let the user know and allow them to switch and connect
• If there's a mismatch during the signing step, instead of only displaying a message, let the user click to switch networks and then connect.