feat: add manage-notification-preferences footer to agent notification emails (#15192)

## Description

Agent notification emails (new conversation, assignment, mention, new
message, SLA misses) currently carry no indication of why the recipient
received them or how to turn them off. When recipients cannot easily
manage these emails, some mark them as spam, which hurts sender
reputation and overall deliverability.

This adds a short footer line to agent notification emails only:

> You're receiving this email because email notifications are enabled
for your account. **Manage notification preferences**.

The link points the recipient to their dashboard profile notification
settings page (`/app/accounts/:account_id/profile/settings`) so they can
disable notifications instead of marking the email as spam. This is a
navigational link only, not a one-click unsubscribe (a real unsubscribe
flow is a separate future change).

Scoping: the mailer layout (`app/views/layouts/mailer/base.liquid`) is
shared by all mailers via `ApplicationMailer`. To keep the footer on
agent notification emails only,
`AgentNotifications::ConversationNotificationsMailer` exposes a
`notification_settings_url` liquid local (built from the existing
`app_account_url` route helper), and the layout renders the footer line
only when that local is present. Transactional and other emails do not
set it, so they are unaffected. The enterprise SLA notification methods
prepend into the same mailer class, so they inherit the footer
automatically.

Part of https://linear.app/chatwoot/issue/CW-7752

## Type of change

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

## How Has This Been Tested?

- `bundle exec rspec
spec/mailers/agent_notifications/conversation_notifications_mailer_spec.rb
spec/mailers/confirmation_instructions_spec.rb` — 26 examples, 0
failures. Asserts the footer link and settings URL are present in an
agent notification email and absent from a non-notification
(confirmation) email.
- `bundle exec rspec
spec/enterprise/mailers/enterprise/agent_notifications/conversation_notifications_mailer_spec.rb`
— 6 examples, 0 failures (SLA notification emails).

## Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [ ] 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

app/mailers/agent_notifications/conversation_notifications_mailer.rb

@@ -57,6 +57,10 @@ class AgentNotifications::ConversationNotificationsMailer < ApplicationMailer
 
   private
 
+  def liquid_locals
+    super.merge({ notification_settings_url: "#{app_account_url(@conversation.account_id)}/profile/settings" })
+  end
+
   def liquid_droppables
     super.merge({
                   user: @agent,

app/views/layouts/mailer/base.liquid

@@ -129,6 +129,14 @@
                 </td>
               </tr>
             {% endif %}
+            {% if notification_settings_url != nil and notification_settings_url != '' %}
+              <tr style="margin: 0;">
+                <td class="footer" style="padding: 8px 8px 0; text-align: center; color: #64748B; font-size: 13px; line-height: 21px;">
+                  You're receiving this email because email notifications are enabled for your account.
+                  <a href="{{ notification_settings_url }}" style="color: #2781F6; font-weight: 600; text-decoration: none;">Manage notification preferences</a>.
+                </td>
+              </tr>
+            {% endif %}
           </table>
         </td>
       </tr>

spec/mailers/agent_notifications/conversation_notifications_mailer_spec.rb

@@ -24,6 +24,11 @@ RSpec.describe AgentNotifications::ConversationNotificationsMailer do
     it 'renders the receiver email' do
       expect(mail.to).to eq([agent.email])
     end
+
+    it 'renders the manage notification preferences footer link' do
+      expect(mail.body.encoded).to match('Manage notification preferences')
+      expect(mail.body.encoded).to include("/app/accounts/#{account.id}/profile/settings")
+    end
   end
 
   describe 'conversation_assignment' do

spec/mailers/confirmation_instructions_spec.rb

@@ -50,6 +50,10 @@ RSpec.describe 'Devise::Mailer' do
       expect(mail.body).not_to include('app/auth/password/edit')
     end
 
+    it 'does not render the agent notification preferences footer' do
+      expect(mail_body).not_to include('Manage notification preferences')
+    end
+
     context 'when there is an inviter' do
       let(:inviter_val) { create(:user, :administrator, skip_confirmation: true, account: account) }