Allow copying the signature link directly instead of only sending it by email (ver. 14.1.0)

Is your feature request related to a problem? Please describe.

When requesting a signature, LibreSign only offers to send the signature link by email. In several situations this is not practical:

• the signer is physically present, or I am already talking to them by phone, chat or instant messaging, and I would like to hand them the link right away;
• I want to forward the link myself, through the channel my client actually uses.

Describe the solution you'd like

In addition to the existing "send by email" action, add an option to generate and copy the signature link directly, for example a "Copy signature link" button next to each signer in the request screen and in the document's detail view. The link would be the same one that is currently embedded in the email, so the existing authentication rules (email token, etc.) would still apply.

Describe alternatives you've considered

• Sending the email to myself and then copying the link out of it — this works but is a clumsy workaround and depends on mail delivery working.
• Rebuilding the URL by hand from the document UUID — not realistic for everyday use.

Additional context

Most commercial e-signature services offer both options (send by email / copy link), and users have come to expect it. This would also make LibreSign usable on instances where email is not set up at all.

Start at the request screen and document detail view, then trace the existing send-by-email action and the signature URL it generates. Done means users can copy the same authenticated signature link directly from both locations without changing the existing email flow or authentication rules.