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/TopBar.tsx

@@ -14,7 +14,7 @@ export function TopBar() {
   const { t } = useTranslation();
   const navigate = useNavigate();
   const { user } = useAuthStore();
-  const { toggleSidebar, toggleAISidebar } = useUIStore();
+  const { toggleSidebar, toggleAISidebar, aiSidebarCollapsed } = useUIStore();
   const { currentTenant, availableTenants, switchTenant, isSwitching } = useTenant();
   const logoutMutation = useLogout();
 
@@ -72,7 +72,10 @@ export function TopBar() {
         {/* AI Assistant toggle */}
         <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"
+          className={clsx(
+            'p-2 rounded-md min-h-touch min-w-touch flex items-center justify-center focus:outline-none focus-visible:ring-2 focus-visible:ring-primary-500',
+            aiSidebarCollapsed ? 'hover:bg-secondary-100' : 'bg-primary-50 text-primary-600 hover:bg-primary-100',
+          )}
           aria-label="KI Assistent"
           title="KI Assistent"
         >