fix: mobile/tablet Geräteerkennung in LoginPage → Redirect zu /mobile/login

- useEffect prüft Bildschirmbreite (<1024px) + User-Agent (Mobi/Android/iPad/iPhone)
- Tablet/Handy wird automatisch zu /mobile/login weitergeleitet
- Desktop bleibt auf /login

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

frontend/src/pages/LoginPage.tsx

@@ -1,10 +1,17 @@
-import { useState } from 'react'
-import { Link, useLocation } from 'react-router-dom'
+import { useState, useEffect } from 'react'
+import { Link, useLocation, useNavigate } from 'react-router-dom'
 import { useAuth } from '../context/AuthContext'
 
 export function LoginPage() {
   const { login }                       = useAuth()
   const location                        = useLocation()
+  const navigate                        = useNavigate()
+
+  // Auf Mobilgeräten / Tablets → mobile Login-Seite
+  useEffect(() => {
+    const isMobile = window.innerWidth < 1024 || /Mobi|Android|Tablet|iPad|iPhone/i.test(navigator.userAgent)
+    if (isMobile) navigate('/mobile/login', { replace: true })
+  }, [])
   const [email, setEmail]               = useState('')
   const [password, setPassword]         = useState('')
   const [error, setError]               = useState('')