feat(ui): move back arrow from topbar to sidebar header on all pages

frontend/src/pages/Help.tsx

@@ -2,7 +2,7 @@ import React, { useState } from 'react';
 import { NavLink, Outlet, useParams } from 'react-router-dom';
 import { useTranslation } from 'react-i18next';
 import { useUIStore } from '@/store/uiStore';
-import { ChevronRight, ChevronDown, BookOpen, Users, Calendar, Mail, FolderOpen, Settings, Shield, Zap, Bot } from 'lucide-react';
+import { ChevronRight, ChevronDown, BookOpen, Users, Calendar, Mail, FolderOpen, Settings, Shield, Zap, Bot, ArrowLeft } from 'lucide-react';
 
 interface HelpNode {
   title: string;
@@ -140,7 +140,17 @@ export function HelpPage() {
     <div className="flex h-full overflow-hidden" data-testid="help-page">
       <aside className={`${sidebarOpen ? 'w-64 border-r border-secondary-200' : 'w-0'} flex-shrink-0 min-h-[calc(100vh-4rem)] transition-all duration-200 overflow-hidden`}>
         <div className="w-64 flex-shrink-0 p-4">
-          <h1 className="text-xl font-bold text-secondary-900 mb-4">Hilfe</h1>
+          <div className="flex items-center gap-2 mb-4">
+            <button
+              onClick={() => window.location.href = '/start'}
+              className="p-1.5 rounded-md text-secondary-400 hover:bg-secondary-100 hover:text-secondary-700 flex items-center justify-center focus:outline-none"
+              aria-label="Zurück zur Startseite"
+              title="Zurück zur Startseite"
+            >
+              <ArrowLeft className="w-4 h-4" />
+            </button>
+            <h1 className="text-xl font-bold text-secondary-900">Hilfe</h1>
+          </div>
           <nav className="space-y-0.5 overflow-y-auto" role="navigation" aria-label="Hilfe Navigation">
             {HELP_TREE.map((node) => (
               <TreeItem key={node.title} node={node} depth={0} />