E-sign templates + field placement (make signatures usable)

From Daniel's walkthrough: signatures are confusing on both firm and client side — how do you set up templates, create a document to sign, and add fields to fill?

What exists today (works, but bare): staff pick a client + a PDF (existing doc or upload), tick the signer contacts, hit Send; Documenso auto-places ONE signature field on page 1 per signer; the client gets a hosted Documenso signing link from the portal.

What's missing (this slice): (1) reusable templates — save a document + its field layout once, reuse per client; (2) a field-placement UI — drag signature/date/initials/text fields onto pages, per signer; (3) signing-order and role controls. Documenso has template + field APIs we can drive from our staff UI, keeping the client experience native.

Also fixes to fold in: the embedded-signing completion listener still listens for DocuSeal postMessages (latent bug), and signing mode is hard-coded to hosted.