feat: initial WooCow plugin — Mailcow/WooCommerce integration

- Mailcow API client wrapping domains, mailboxes, aliases endpoints
- Admin backend: server management, customer-domain assignments, mailbox overview
- WooCommerce My Account: email hosting tab with mailbox/alias management
- Per-mailbox password change (independent of WP account password)
- Optional WP account password sync to all customer mailboxes
- Installer creates wp_woocow_servers and wp_woocow_assignments DB tables
- Full nonce + capability + ownership verification on all AJAX endpoints

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

includes/class-woocow-pw-sync.php

@@ -0,0 +1,51 @@
+<?php
+/**
+ * Adds an optional "sync password to mailboxes" checkbox on the
+ * WooCommerce My Account > Account Details page.
+ *
+ * Loaded automatically from woocow.php when WooCommerce is active.
+ */
+defined( 'ABSPATH' ) || exit;
+
+class WooCow_PW_Sync {
+
+    public function __construct() {
+        // Render the checkbox below the password fields
+        add_action( 'woocommerce_edit_account_form', [ $this, 'render_checkbox' ] );
+    }
+
+    public function render_checkbox(): void {
+        // Only show if the current customer has mailcow assignments
+        global $wpdb;
+        $count = (int) $wpdb->get_var( $wpdb->prepare(
+            "SELECT COUNT(*) FROM {$wpdb->prefix}woocow_assignments WHERE customer_id = %d",
+            get_current_user_id()
+        ) );
+
+        if ( ! $count ) {
+            return;
+        }
+        ?>
+        <fieldset class="woocow-pw-sync-field">
+            <legend><?php esc_html_e( 'Email Hosting', 'woocow' ); ?></legend>
+            <label for="woocow_sync_pw">
+                <input type="checkbox" name="woocow_sync_pw" id="woocow_sync_pw" value="1">
+                <?php esc_html_e( 'Also update password for all my email mailboxes (only applies when changing password above)', 'woocow' ); ?>
+            </label>
+        </fieldset>
+        <style>
+        .woocow-pw-sync-field {
+            border: 1px dashed #ddd;
+            border-radius: 6px;
+            padding: 12px 16px;
+            margin-top: 20px;
+        }
+        .woocow-pw-sync-field legend {
+            font-weight: 700;
+            padding: 0 6px;
+            color: #2271b1;
+        }
+        </style>
+        <?php
+    }
+}