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

frontend/src/pages/StartPage.tsx

@@ -8,7 +8,7 @@ import { useNavigate } from 'react-router-dom';
 import { useTranslation } from 'react-i18next';
 import { useAuthStore } from '@/store/authStore';
 import { useUIStore } from '@/store/uiStore';
-import { Settings, HelpCircle, LayoutGrid, ArrowRight, Plus, Bot, Zap } from 'lucide-react';
+import { Settings, HelpCircle, LayoutGrid, ArrowRight, Plus, Bot, Zap, ArrowLeft } from 'lucide-react';
 
 // Workspace tile definition
 interface WorkspaceTile {
@@ -49,7 +49,15 @@ export function StartPage() {
     <div className="flex h-full overflow-hidden bg-secondary-50">
       {/* Left column — menu (toggleable via hamburger) */}
       <aside className={`${sidebarOpen ? 'w-56' : 'w-0'} bg-white border-r border-secondary-200 flex flex-col flex-shrink-0 transition-all duration-200 overflow-hidden`}>
-        <div className="h-[58px] flex items-center px-4 border-b border-secondary-200 w-56 flex-shrink-0">
+        <div className="h-[58px] flex items-center gap-2 px-4 border-b border-secondary-200 w-56 flex-shrink-0">
+          <button
+            onClick={() => navigate('/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 focus-visible:ring-2 focus-visible:ring-primary-500"
+            aria-label="Zurück zur Startseite"
+            title="Zurück zur Startseite"
+          >
+            <ArrowLeft className="w-4 h-4" />
+          </button>
           <span className="text-xl font-bold text-secondary-900">leocrm</span>
         </div>
         <nav className="flex-1 overflow-y-auto py-4 w-56 flex-shrink-0">