Copy invite link in the Members dashboard

Pending organization invitations in the dashboard’s Members page get a “Copy invite link” action that copies <server>/auth/invitations/<token> to the clipboard, matching what the CLI already prints. The invite modal also now has a confirmation step that surfaces the acceptance link, plus a subtitle that calls out whether an email was actually sent. This unblocks self-hosted organizations running without SMTP or Mailgun, where invitations used to be created successfully but the invitee had no way to receive the link.