feat: comprehensive UI/UX and feature improvements

- Dark mode: CSS custom properties (global.css) + toggle button, persisted in localStorage
- i18n: German/English language switcher (lang/de.php, lang/en.php, includes/I18n.php)
- Mobile-responsive admin layout: hamburger menu, sidebar overlay (global.js + global.css)
- Shared admin navigation include (includes/admin_nav.php) used across all admin pages
- Toast notifications system globally available via global.js
- Voucher templates/profiles: CRUD UI at admin/templates.php with voucher_templates DB table
- Bulk voucher creation: create 1-20 vouchers at once with multi-print layout on index.php
- Configurable voucher defaults: expire time, device limit, max limit in admin settings
- Template quick-select on voucher form: auto-fills max_uses and expire_minutes
- Password reset flow: forgot_password.php + reset_password.php with token-based reset
- Audit log UI: admin/audit_log.php with filter, pagination, audit_log DB table
- Audit logging on login, user create/edit/delete, site create/edit/delete
- Admin pages updated: index, vouchers, users, sites all use admin_nav.php + dark mode + i18n
- Voucher admin: live search input added alongside existing status filter + pagination
- Users admin: password-reset-link button per user row (when SMTP enabled)
- Login page: i18n, dark mode, language switcher, forgot password link

https://claude.ai/code/session_01YN6Bcm1VSi8mpDeyKpyrdJ

includes/Auth.php

@@ -38,6 +38,7 @@ class Auth {
             $this->clearLoginAttempts($ip, $email);
             $this->setUserSession($user);
             $this->updateLastLogin($user['id']);
+            $this->writeAuditLog($user['id'], 'user_login', 'user', $user['id'], 'Login erfolgreich');
             return true;
         }
 
@@ -45,6 +46,17 @@ class Auth {
         return false;
     }
 
+    public function writeAuditLog($userId, $action, $entityType = null, $entityId = null, $details = null) {
+        try {
+            $this->db->execute(
+                "INSERT INTO audit_log (user_id, action, entity_type, entity_id, details, ip_address) VALUES (?, ?, ?, ?, ?, ?)",
+                [$userId, $action, $entityType, $entityId !== null ? (string)$entityId : null, $details, $_SERVER['REMOTE_ADDR'] ?? '']
+            );
+        } catch (\Exception $e) {
+            // audit_log table may not exist on old installs
+        }
+    }
+
     private function isRateLimited($ip, $email) {
         try {
             $count = $this->db->fetchOne(