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/I18n.php

@@ -0,0 +1,53 @@
+<?php
+
+class I18n {
+    private static array $translations = [];
+    private static string $language = 'de';
+    private static bool $initialized = false;
+
+    public static function init(): void {
+        if (self::$initialized) return;
+
+        if (!isset($_SESSION)) {
+            if (session_status() === PHP_SESSION_NONE) session_start();
+        }
+
+        if (isset($_GET['set_lang']) && in_array($_GET['set_lang'], ['de', 'en'], true)) {
+            $_SESSION['language'] = $_GET['set_lang'];
+        }
+
+        self::$language = $_SESSION['language'] ?? 'de';
+
+        $langFile = __DIR__ . '/../lang/' . self::$language . '.php';
+        if (file_exists($langFile)) {
+            self::$translations = require $langFile;
+        } else {
+            $fallback = __DIR__ . '/../lang/de.php';
+            if (file_exists($fallback)) {
+                self::$translations = require $fallback;
+            }
+        }
+
+        self::$initialized = true;
+    }
+
+    public static function t(string $key, array $replace = []): string {
+        $text = self::$translations[$key] ?? $key;
+        foreach ($replace as $k => $v) {
+            $text = str_replace('{' . $k . '}', (string)$v, $text);
+        }
+        return $text;
+    }
+
+    public static function getLanguage(): string {
+        return self::$language;
+    }
+
+    public static function getAvailable(): array {
+        return ['de' => 'Deutsch', 'en' => 'English'];
+    }
+}
+
+function __($key, array $replace = []): string {
+    return I18n::t($key, $replace);
+}