refactor(config): remove redundant wallet field from WalletConfigInner

refactor(config): remove redundant wallet field from WalletConfigInner

`WalletConfigInner` duplicated its own wallet name in a `wallet` field,
even though it's already the key in `WalletConfig::wallets` - this
showed up as a redundant `"wallet"` entry in `wallets list` output.

`TryFrom<&WalletConfigInner> for WalletOpts` relied on that field, so
it's replaced with a private `to_wallet_opts(&self, wallet_name: &str)`
that takes the name from the caller's map lookup instead. Old config.toml
files still load fine; serde ignores the now-unknown `wallet` key.

Fixes #332