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>
   );