feat: complete Newsletter Campaigns Phase 1

- Add default campaign email template to DefaultTemplates.php
- Add toggle settings (campaign_scheduling, subscriber_limit_enabled)
- Add public unsubscribe endpoint with secure token verification
- Update CampaignManager to use NewsletterController unsubscribe URLs
- Add generate_unsubscribe_url() helper for email templates

includes/Email/DefaultTemplates.php

@@ -90,6 +90,7 @@ class DefaultTemplates
                 'order_cancelled' => self::customer_order_cancelled(),
                 'order_refunded' => self::customer_order_refunded(),
                 'new_customer' => self::customer_new_customer(),
+                'newsletter_campaign' => self::customer_newsletter_campaign(),
             ],
             'staff' => [
                 'order_placed' => self::staff_order_placed(),
@@ -139,6 +140,7 @@ class DefaultTemplates
                 'order_cancelled' => 'Order #{order_number} has been cancelled',
                 'order_refunded' => 'Refund processed for order #{order_number}',
                 'new_customer' => 'Welcome to {site_name}! 🎁 Exclusive offer inside',
+                'newsletter_campaign' => '{campaign_title}',
             ],
             'staff' => [
                 'order_placed' => '[NEW ORDER] #{order_number} - ${order_total} from {customer_name}',
@@ -206,6 +208,30 @@ Got questions? Our customer service team is ready to help: {support_email}
 [/card]';
     }
 
+    /**
+     * Customer: Newsletter Campaign
+     * Master design template for newsletter campaigns
+     * The {content} variable is replaced with the actual campaign content
+     */
+    private static function customer_newsletter_campaign()
+    {
+        return '[card type="hero"]
+## {campaign_title}
+[/card]
+
+[card]
+{content}
+[/card]
+
+[card type="basic" bg="#f5f5f5"]
+You are receiving this because you subscribed to {site_name} newsletter.
+
+[Unsubscribe]({unsubscribe_url}) | [Visit Store]({site_url})
+
+© {current_year} {site_name}. All rights reserved.
+[/card]';
+    }
+
     /**
      * Customer: Order Placed
      * Sent immediately when customer places an order