Custom Signet support

[1ma]

Describe the new feature or improvement you would like to see in the Envoy mobile app.

Being able to connect to signet networks other than Mutiny's

Explain the specific use case or scenario where this feature would be valuable to users.

One of the key features of signet is that anyone can run it's own. It'd be cool if Envoy was able to connect to any given Electrum server. As it is right now the custom Electrum feature is only enabled on mainnet.

Ideas for implementation

Envoy already has UI to customize the Electrum server, but it's only used on mainnet. On signet and testnet it seems to be hardcoded:

envoy/lib/business/account_manager.dart

Line 162 in b6198fa

server = Settings.MUTINYNET_ELECTRUM_SERVER;

In testnet the hardcoding is not a big deal because there is just one testnet network. In signet it means you're unable to use any signet network other than Mutiny's.

Screenshots/Design

No response

Additional Context

No response