Simplified party & signatory handling

We’re streamlining how parties and signatories are created and managed in fynk. Adding a counterparty used to start with an up-front choice of “entity type” (person, business, or to-be-determined), which added friction and rarely matched how people actually think about a contract. That distinction is gone. A party is now simply a label (its role in the document) plus, optionally, the legal entity name and its signatories.