On larger documents we could accidentally start trying to render fields
while not all pages of the PDF have loaded due to us checking for a
single page existing. This would cause an error to be thrown, hard
locking those documents.
This change resolves this by grabbing the highest page number from the
given fields and using it for the visibility check instead.
Introduces the ability for users with the **Assistant** role to prefill
fields on behalf of other signers. Assistants can fill in various field
types such as text, checkboxes, dates, and more, streamlining the
document preparation process before it reaches the final signers.
