Fix: Livechat offline form allows submission without a configured recipient email

Description:

The Livechat offline form remains visible and allows visitors to submit messages even when no email address is configured to receive them. This leads to a broken UX where visitors believe their messages have been sent, but they are never delivered to anyone.

// TODO Block offline form if Livechat_offline_email is undefined
// (it does not make sense to have an offline form that does nothing)

Steps to reproduce:

1. Click the settings icon > Omnichannel > Livechat Appearance > Omnichannel offline section (MUST be admin)
2. Enable Display offline form.
3. Leave the Email address to send offline messages field empty (here the problem)
4. Open the Livechat widget (http://localhost:3000/livechat) as a visitor when no agents are online.
5. Fill out the offline form and click Send

Expected behavior:

• The offline form should not be visible if there is no destination for the message.
• The visitor should see a message indicating that the offline form is unavailable.

Actual behavior:

• The form is displayed normally.
• The visitor can submit the message.

Server Setup Information:

• Operating System: Linux
• NodeJS Version: 22.16.0

Start at the Livechat widget's offline form entry point and inspect how the Livechat_offline_email setting is read. Reproduce the empty-recipient scenario from the listed Omnichannel settings, then verify that visitors cannot use the form and receive an unavailable-state message when no recipient email is configured.