feat(13-email-mailboxes): phase 13 complete — email DB foundation + SMTP mailbox CRUD

3 migrations (email_mailboxes, email_templates, email_logs), full CRUD
for SMTP mailboxes with encrypted passwords (IntegrationSecretCipher),
native SMTP connection test via stream_socket_client, sidebar navigation.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

resources/views/layouts/app.php

@@ -95,6 +95,9 @@
             <a class="sidebar__sublink<?= $currentMenu === 'settings' && $currentSettings === 'accounting' ? ' is-active' : '' ?>" href="/settings/accounting">
               <?= $e($t('navigation.accounting')) ?>
             </a>
+            <a class="sidebar__sublink<?= $currentMenu === 'settings' && $currentSettings === 'email-mailboxes' ? ' is-active' : '' ?>" href="/settings/email-mailboxes">
+              Skrzynki pocztowe
+            </a>
           </div>
         </details>
       </nav>