feat: add standalone AI assistant window button

- Add ExternalLink button to AI assistant toolbar
- New route /ai-assistant-standalone renders AIAssistantPage without sidebar
- Opens in new browser window via window.open

frontend/src/pages/AIAssistant.tsx

@@ -4,7 +4,7 @@ import { ChatWindow } from '@/components/ai/ChatWindow';
 import { ResizablePanel } from '@/components/ui/ResizablePanel';
 import { usePluginToolbarStore } from '@/store/pluginToolbarStore';
 import { fetchAgents, createSession, type AIAgent } from '@/api/ai';
-import { ChevronLeft, Folder, Plus } from 'lucide-react';
+import { ChevronLeft, ExternalLink, Folder, Plus } from 'lucide-react';
 
 export function AIAssistantPage() {
   const [activeSessionId, setActiveSessionId] = useState<string | null>(null);
@@ -67,6 +67,19 @@ export function AIAssistantPage() {
           }
         },
       },
+      {
+        id: 'open-standalone',
+        plugin: 'ai_assistant',
+        label: 'In neuem Fenster',
+        type: 'button',
+        group: 'actions',
+        icon: (
+          <ExternalLink className="w-3.5 h-3.5" strokeWidth={2} />
+        ),
+        onClick: () => {
+          window.open('/ai-assistant-standalone', '_blank', 'width=800,height=600');
+        },
+      },
     ]);
     return () => unregisterPlugin('ai_assistant');
   }, [agents, selectedAgentId, registerItems, unregisterPlugin]);

frontend/src/pages/AIAssistantStandalone.tsx

@@ -0,0 +1,10 @@
+import React from 'react';
+import { AIAssistantPage } from './AIAssistant';
+
+export function AIAssistantStandalonePage() {
+  return (
+    <div className="h-screen w-screen overflow-hidden bg-white">
+      <AIAssistantPage />
+    </div>
+  );
+}