fix: redirect to dashboard after successful login

LoginScreen was not navigating after login, leaving users on empty page.
Now explicitly redirects to '/' (dashboard) after successful login.

This fixes the "empty page after first login" issue.

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

frontend/src/pages/LoginScreen.jsx

@@ -33,6 +33,8 @@ export default function LoginScreen() {
     setLoading(true); setError(null)
     try {
       await login({ email: email.trim().toLowerCase(), password: password })
+      // Redirect to dashboard after successful login
+      window.location.href = '/'
     } catch(e) {
       setError(e.message || 'Ungültige E-Mail oder Passwort')
     } finally { setLoading(false) }