Function resolve_address_book 

pub fn resolve_address_book(
    input: &str,
    config: &Config,
) -> Result<ResolvedInput, String>

Resolves a user-supplied input string against the address book.

Supports @label shortcuts (e.g. @main-wallet) and direct address matching. Returns the original input unchanged if no match is found.

Errors

Returns a user-facing error string when @label is used but the label doesn’t exist in the address book — this should be returned as a 400.