feat: window management system with minimize, fullscreen, and AI chat split

- Window manager store (Zustand) for managing open/minimized/fullscreen windows
- Window component with header controls: KI-Chat toggle, fullscreen, minimize, close
- AI chat panel with streaming chat via existing /ai/sessions API
- WindowContainer renders all non-minimized windows
- TopBar shows minimized windows as clickable pills
- ContactEditForm extracted from ContactEditModal for window rendering
- ContactsList and ContactDetailPage use openWindow() instead of modal state
- Draggable windows with z-index management

frontend/src/components/layout/AppShell.tsx

@@ -9,6 +9,7 @@ import { useAIContext } from '@/hooks/useAIContext';
 import { useAIUIControl } from '@/hooks/useAIUIControl';
 import { PluginRegistry } from '@/components/plugins/PluginRegistry';
 import { AIUIControlIndicator } from '@/components/ai-ui-control/AIUIControlIndicator';
+import { WindowContainer } from '@/components/window/WindowContainer';
 
 export function AppShell() {
   const location = useLocation();
@@ -44,6 +45,7 @@ export function AppShell() {
       {/* Message Sidebar — full height, right of TopBar and Toolbar */}
       {showMessageSidebar && <MessageSidebar />}
       <AIUIControlIndicator />
+      <WindowContainer />
       <ToastContainer />
     </div>
   );

frontend/src/components/layout/TopBar.tsx

@@ -8,7 +8,8 @@ import { useLogout } from '@/api/hooks';
 import { Avatar } from '@/components/ui/Avatar';
 import { SearchDropdown } from '@/components/shared/SearchDropdown';
 import { SuggestionBadge } from '@/components/ai/SuggestionBadge';
-import { Building, ChevronDown, Menu, Zap, Bot } from 'lucide-react';
+import { Building, ChevronDown, Menu, Zap, Bot, Layers } from 'lucide-react';
+import { useWindowStore } from '@/store/windowStore';
 
 export function TopBar() {
   const { t } = useTranslation();
@@ -17,6 +18,8 @@ export function TopBar() {
   const tenants = user?.tenants || [];
   const { toggleSidebar, toggleMessageSidebar } = useUIStore();
   const logoutMutation = useLogout();
+  const minimizedWindows = useWindowStore((s) => s.windows.filter((w) => w.state === 'minimized'));
+  const restoreWindow = useWindowStore((s) => s.restoreWindow);
 
   const [userMenuOpen, setUserMenuOpen] = useState(false);
   const userRef = useRef<HTMLDivElement>(null);
@@ -77,6 +80,22 @@ export function TopBar() {
       </div>
 
       <div className="flex items-center gap-2">
+        {/* Minimized windows */}
+        {minimizedWindows.length > 0 && (
+          <div className="flex items-center gap-1.5">
+            {minimizedWindows.map((win) => (
+              <button
+                key={win.id}
+                onClick={() => restoreWindow(win.id)}
+                className="flex items-center gap-1.5 bg-secondary-100 text-secondary-700 px-3 py-1 rounded-md text-sm hover:bg-secondary-200"
+                title={win.title}
+              >
+                <Layers className="w-3.5 h-3.5" />
+                <span className="max-w-32 truncate">{win.title}</span>
+              </button>
+            ))}
+          </div>
+        )}
         {/* User menu */}
         <div ref={userRef} className="relative">
           <button