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

frontend/src/components/layout/Sidebar.tsx

@@ -1,9 +1,9 @@
 import React, { useState, useEffect, useMemo } from 'react';
 import clsx from 'clsx';
-import { NavLink, useLocation } from 'react-router-dom';
+import { NavLink, useLocation, useNavigate } from 'react-router-dom';
 import { useTranslation } from 'react-i18next';
 import { useUIStore } from '@/store/uiStore';
-import { ChevronRight, FileText, Home, Settings, Users } from 'lucide-react';
+import { ChevronRight, FileText, Home, Settings, Users, ArrowLeft } from 'lucide-react';
 import { usePluginStore } from '@/store/pluginStore';
 // Curated icon map — avoids `import * as LucideIcons` which loads ALL icons and causes OOM in tests
 import {
@@ -58,6 +58,7 @@ const singleItems: NavSingleItem[] = [
 const bottomItems: NavSingleItem[] = [];
 
 export function Sidebar() {
+  const navigate = useNavigate();
   const { t } = useTranslation();
   const { sidebarOpen, setSidebarOpen } = useUIStore();
   const location = useLocation();
@@ -178,7 +179,15 @@ export function Sidebar() {
         aria-label="Seitenleiste Navigation"
         data-testid="sidebar"
       >
-        <div className="h-[58px] flex items-center px-4 border-b border-secondary-700">
+        <div className="h-[58px] flex items-center gap-2 px-4 border-b border-secondary-700">
+          <button
+            onClick={() => navigate('/start')}
+            className="p-1.5 rounded-md text-secondary-400 hover:bg-secondary-700 hover:text-white flex items-center justify-center focus:outline-none"
+            aria-label="Zurück zur Startseite"
+            title="Zurück zur Startseite"
+          >
+            <ArrowLeft className="w-4 h-4" />
+          </button>
           <span className="text-xl font-bold text-white">leocrm</span>
         </div>
         <nav className="flex-1 overflow-y-auto py-4" aria-label="Hauptnavigation">