feat: Add admin toggle and dark mode to login page

- Add admin toggle switch to switch between user/admin login modes
- Implement dark mode toggle button in top-right corner
- Style dark mode toggle with adaptive colors and hover effects

Enhances login page with flexible authentication modes and
improved visual customization options for better user experience.

frontend/src/pages/LoginPage.js

@@ -1,6 +1,6 @@
 import React, { useState } from 'react';
-import { Form, Input, Button, Typography, Alert, message } from 'antd';
-import { LockOutlined, UserOutlined } from '@ant-design/icons';
+import { Form, Input, Button, Typography, Alert, message, Switch } from 'antd';
+import { LockOutlined, UserOutlined, SettingOutlined, SunOutlined, MoonOutlined } from '@ant-design/icons';
 import { preserveFullUrl } from '../utils/urlUtils';
 
 const { Title } = Typography;
@@ -9,7 +9,30 @@ const { Title } = Typography;
 export default function LoginPage({ onLogin }) {
   const [loading, setLoading] = useState(false);
   const [error, setError] = useState(null);
-  const darkMode = typeof window !== 'undefined' && localStorage.getItem('darkMode') === 'true';
+  const [isAdminMode, setIsAdminMode] = useState(false);
+  const [darkMode, setDarkMode] = useState(
+    typeof window !== 'undefined' && localStorage.getItem('darkMode') === 'true'
+  );
+
+  const toggleDarkMode = () => {
+    const newDarkMode = !darkMode;
+    setDarkMode(newDarkMode);
+    localStorage.setItem('darkMode', newDarkMode.toString());
+    
+    // Update HTML class for immediate visual feedback
+    const htmlElement = document.documentElement;
+    const bodyElement = document.body;
+    
+    if (newDarkMode) {
+      htmlElement.classList.add('dark-mode');
+      htmlElement.style.backgroundColor = '#181818';
+      bodyElement.style.backgroundColor = '#181818';
+    } else {
+      htmlElement.classList.remove('dark-mode');
+      htmlElement.style.backgroundColor = '#f5f5f5';
+      bodyElement.style.backgroundColor = '#f5f5f5';
+    }
+  };
 
   const onFinish = async (values) => {
     setLoading(true);
@@ -53,8 +76,27 @@ export default function LoginPage({ onLogin }) {
         display: 'flex',
         alignItems: 'center',
         justifyContent: 'center',
+        position: 'relative'
       }}
     >
+      {/* Dark Mode Toggle in der oberen rechten Ecke */}
+      <Button
+        type="text"
+        shape="circle"
+        icon={darkMode ? <SunOutlined /> : <MoonOutlined />}
+        onClick={toggleDarkMode}
+        style={{
+          position: 'absolute',
+          top: 16,
+          right: 16,
+          color: darkMode ? '#fff' : '#666',
+          backgroundColor: darkMode ? 'rgba(255,255,255,0.1)' : 'rgba(0,0,0,0.1)',
+          border: 'none',
+          width: 40,
+          height: 40
+        }}
+        title={darkMode ? 'Light Mode aktivieren' : 'Dark Mode aktivieren'}
+      />
       <div
         style={{
           width: 350,
@@ -62,20 +104,61 @@ export default function LoginPage({ onLogin }) {
           background: darkMode ? '#1f1f1f' : '#fff',
           borderRadius: 8,
           boxShadow: darkMode ? '0 2px 8px #222' : '0 2px 8px #eee',
-          color: darkMode ? '#fff' : undefined
+          color: darkMode ? '#fff' : '#1f1f1f'
         }}
       >
-        <Title level={3} style={{ textAlign: 'center', color: darkMode ? '#fff' : undefined }}>Anmeldung</Title>
+        <Title level={3} style={{ textAlign: 'center', color: darkMode ? '#fff' : '#1f1f1f' }}>
+          {isAdminMode ? 'Admin-Anmeldung' : 'Benutzer-Anmeldung'}
+        </Title>
+        
+        {/* Admin-Mode Toggle */}
+        <div style={{ 
+          display: 'flex', 
+          alignItems: 'center', 
+          justifyContent: 'center', 
+          marginBottom: 24,
+          gap: 8
+        }}>
+          <SettingOutlined style={{ color: darkMode ? '#888' : '#666' }} />
+          <span style={{ color: darkMode ? '#888' : '#666', fontSize: 14 }}>Admin-Login</span>
+          <Switch 
+            checked={isAdminMode}
+            onChange={setIsAdminMode}
+            size="small"
+          />
+        </div>
+
         {error && <Alert type="error" message={error} showIcon style={{ marginBottom: 16 }} />}
         <Form name="login" onFinish={onFinish} layout="vertical">
-          <Form.Item name="username" label={<span style={{ color: darkMode ? '#fff' : undefined }}>Benutzername (nur Admin)</span>} >
-            <Input prefix={<UserOutlined />} placeholder="Benutzername" autoComplete="username" style={darkMode ? { background: '#222', color: '#fff' } : {}} />
-          </Form.Item>
-          <Form.Item name="password" label={<span style={{ color: darkMode ? '#fff' : undefined }}>Passwort</span>} rules={[{ required: true, message: 'Bitte Passwort eingeben!' }]}> 
-            <Input.Password prefix={<LockOutlined />} placeholder="Passwort" autoComplete="current-password" style={darkMode ? { background: '#222', color: '#fff' } : {}} />
+          {/* Benutzername-Feld nur im Admin-Mode */}
+          {isAdminMode && (
+            <Form.Item 
+              name="username" 
+              label={<span style={{ color: darkMode ? '#fff' : '#1f1f1f' }}>Benutzername</span>}
+              rules={[{ required: isAdminMode, message: 'Bitte Benutzername eingeben!' }]}
+            >
+              <Input 
+                prefix={<UserOutlined />} 
+                placeholder="Admin-Benutzername" 
+                autoComplete="username" 
+                style={darkMode ? { background: '#222', color: '#fff' } : {}} 
+              />
+            </Form.Item>
+          )}
+          <Form.Item 
+            name="password" 
+            label={<span style={{ color: darkMode ? '#fff' : '#1f1f1f' }}>Passwort</span>} 
+            rules={[{ required: true, message: 'Bitte Passwort eingeben!' }]}
+          > 
+            <Input.Password 
+              prefix={<LockOutlined />} 
+              placeholder={isAdminMode ? "Admin-Passwort" : "Passwort"} 
+              autoComplete="current-password" 
+              style={darkMode ? { background: '#222', color: '#fff' } : {}} 
+            />
           </Form.Item>
           <Button type="primary" htmlType="submit" block loading={loading}>
-            Einloggen
+            {isAdminMode ? 'Als Admin einloggen' : 'Einloggen'}
           </Button>
         </Form>
       </div>