feat: AI sidebar full height with collapsed icon strip and drag resize

- AISidebar: full height, right of TopBar and Toolbar (all pages except AI Assistant)
- AISidebar: collapsed mode shows narrow 48px icon strip with KI button
- AISidebar: expanded mode uses ResizablePanel with drag-to-resize (240-600px)
- AppShell: moved AISidebar outside inner content container to full-height flex
- uiStore: renamed aiSidebarOpen→aiSidebarCollapsed (default: true=collapsed)
- TopBar: AI button highlights when sidebar is expanded

frontend/src/components/layout/AISidebar.tsx

@@ -5,7 +5,7 @@ import { createSession, fetchSessions } from '@/api/ai';
 import { useUIStore } from '@/store/uiStore';
 
 export function AISidebar() {
-  const { setAISidebarOpen } = useUIStore();
+  const { aiSidebarCollapsed, toggleAISidebar } = useUIStore();
   const [sessionId, setSessionId] = useState<string | null>(null);
   const [loading, setLoading] = useState(true);
 
@@ -27,6 +27,28 @@ export function AISidebar() {
     })();
   }, []);
 
+  // Collapsed: narrow icon strip
+  if (aiSidebarCollapsed) {
+    return (
+      <div
+        className="flex-shrink-0 w-12 flex flex-col items-center border-l border-secondary-200 bg-white py-3"
+        data-testid="ai-sidebar-collapsed"
+      >
+        <button
+          onClick={toggleAISidebar}
+          className="p-2 rounded-md hover:bg-secondary-100 min-h-touch min-w-touch flex items-center justify-center focus:outline-none focus-visible:ring-2 focus-visible:ring-primary-500"
+          aria-label="KI Assistent öffnen"
+          title="KI Assistent"
+        >
+          <svg className="w-5 h-5 text-secondary-600" fill="none" viewBox="0 0 24 24" stroke="currentColor" aria-hidden="true">
+            <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M9.75 17L9 20l-1 1h8l-1-1-.75-3M3 13h18M5 17h14a2 2 0 002-2V5a2 2 0 00-2-2H5a2 2 0 00-2 2v10a2 2 0 002 2z" />
+          </svg>
+        </button>
+      </div>
+    );
+  }
+
+  // Expanded: full chat panel with resize handle
   return (
     <ResizablePanel
       initialWidth={320}
@@ -39,11 +61,14 @@ export function AISidebar() {
         <div className="h-12 flex items-center justify-between px-3 border-b border-secondary-200">
           <span className="text-sm font-medium text-secondary-700">KI Assistent</span>
           <button
-            onClick={() => setAISidebarOpen(false)}
-            className="text-secondary-400 hover:text-secondary-600"
-            title="Schließen"
+            onClick={toggleAISidebar}
+            className="p-1 rounded text-secondary-400 hover:text-secondary-600 hover:bg-secondary-100"
+            title="Einklappen"
+            aria-label="KI Assistent einklappen"
           >
-            ✕
+            <svg className="w-4 h-4" fill="none" viewBox="0 0 24 24" stroke="currentColor" aria-hidden="true">
+              <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M9 5l7 7-7 7" />
+            </svg>
           </button>
         </div>
         <div className="flex-1 overflow-hidden">