feat(cloud): Add support for viewing status of SSL in custom domains (#12011)

# Pull Request Template

## Description

Fixes
[CW-4620](https://linear.app/chatwoot/issue/CW-4620/rethinking-custom-domains-in-chatwoot)

<img width="642" height="187" alt="Screenshot 2025-07-29 at 8 17 44 PM"
src="https://github.com/user-attachments/assets/ad2f5dac-4b27-4dce-93ca-6cbba74443fb"
/>


## Type of change

- [x] New feature (non-breaking change which adds functionality)

## How Has This Been Tested?



## Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [x] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

---------

Co-authored-by: Vishnu Narayanan <iamwishnu@gmail.com>
Co-authored-by: Pranav <pranavrajs@gmail.com>
Co-authored-by: Pranav <pranav@chatwoot.com>

app/mailers/portal_instructions_mailer.rb

@@ -0,0 +1,41 @@
+class PortalInstructionsMailer < ApplicationMailer
+  def send_cname_instructions(portal:, recipient_email:)
+    return unless smtp_config_set_or_development?
+    return if target_domain.blank?
+
+    @portal = portal
+    @cname_record = generate_cname_record
+
+    send_mail_with_liquid(
+      to: recipient_email,
+      subject: I18n.t('portals.send_instructions.subject', custom_domain: @portal.custom_domain)
+    )
+  end
+
+  private
+
+  def liquid_locals
+    { cname_record: @cname_record }
+  end
+
+  def generate_cname_record
+    "#{@portal.custom_domain} CNAME #{target_domain}"
+  end
+
+  def target_domain
+    helpcenter_url = ENV.fetch('HELPCENTER_URL', '')
+    frontend_url = ENV.fetch('FRONTEND_URL', '')
+
+    return extract_hostname(helpcenter_url) if helpcenter_url.present?
+    return extract_hostname(frontend_url) if frontend_url.present?
+
+    ''
+  end
+
+  def extract_hostname(url)
+    uri = URI.parse(url)
+    uri.host
+  rescue URI::InvalidURIError
+    url.gsub(%r{https?://}, '').split('/').first
+  end
+end