Add Hallinta module with iRedMail email management

New "Hallinta" main tab (superadmin only) with "Sähköposti" sub-tab for
managing email via iRedAdmin-Pro REST API. Features:
- IRedMailClient PHP class with cookie-based session auth + auto-retry
- Domain CRUD (list, create, delete)
- Mailbox CRUD (list, create, delete, password change)
- Alias CRUD (list, create, delete)
- Configuration modal (API URL, admin credentials, connection test)
- Search/filter for mailboxes
- 13 new API endpoints, all requireSuperAdmin()

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

style.css

@@ -2431,3 +2431,8 @@ span.empty {
 .integration-config-card {
     border-left: 3px solid var(--primary-color);
 }
+
+/* iRedMail */
+#iredmail-status { border-radius: 8px; transition: background 0.3s; }
+.btn-danger { background: #e74c3c; color: #fff; border: none; border-radius: 4px; cursor: pointer; padding: 4px 10px; font-size: 0.82rem; }
+.btn-danger:hover { background: #c0392b; }