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/AppShell.tsx

@@ -8,11 +8,10 @@ import { ToastContainer } from '@/components/ui/Toast';
 import { useUIStore } from '@/store/uiStore';
 
 export function AppShell() {
-  const { aiSidebarOpen } = useUIStore();
   const location = useLocation();
 
   // Hide AI sidebar on the AI Assistant page itself
-  const showAISidebar = aiSidebarOpen && !location.pathname.startsWith('/ai-assistant');
+  const showAISidebar = !location.pathname.startsWith('/ai-assistant');
 
   return (
     <div className="flex h-screen overflow-hidden bg-secondary-50" data-testid="app-shell">
@@ -30,9 +29,10 @@ export function AppShell() {
           >
             <Outlet />
           </main>
-          {showAISidebar && <AISidebar />}
         </div>
       </div>
+      {/* AI Sidebar — full height, right of TopBar and Toolbar */}
+      {showAISidebar && <AISidebar />}
       <ToastContainer />
     </div>
   );