feat(admin): copy buttons and shared clipboard helper

- Add magesail-clipboard.js (clipboard API + execCommand fallback)
- Tunnel: copy store code, key, Holesail command, log tail; wire polling UI
- How-To: copy per code block; Howto block + magesail-howto-copy.js
- NGINX map: copy raw map from textarea; translations in Map block
- Document copy UX in docs/admin-ui.md

Magesail/Block/Adminhtml/Howto.php

@@ -0,0 +1,31 @@
+<?php
+
+declare(strict_types=1);
+
+namespace MageSail\Magesail\Block\Adminhtml;
+
+use Magento\Backend\Block\Template;
+use Magento\Backend\Block\Template\Context;
+use Magento\Framework\Serialize\Serializer\Json;
+
+class Howto extends Template
+{
+    public function __construct(
+        Context $context,
+        private readonly Json $json,
+        array $data = []
+    ) {
+        parent::__construct($context, $data);
+    }
+
+    public function getHowtoCopyInitJson(): string
+    {
+        return $this->json->serialize([
+            'translations' => [
+                'copy' => (string) __('Copy'),
+                'copied' => (string) __('Copied!'),
+                'copyFailed' => (string) __('Copy failed'),
+            ],
+        ]);
+    }
+}