feat(AppShell): add SidebarTrigger component for improved sidebar accessibility

src/renderer/components/layout/AppShell.tsx

@@ -29,6 +29,7 @@ import {
   SidebarMenuButton,
   SidebarMenuItem,
   SidebarProvider,
+  SidebarTrigger,
   useSidebar,
 } from '@/components/ui/sidebar';
 import {
@@ -129,6 +130,9 @@ function AppShellInner({ children }: AppShellProps) {
           />
         ) : (
           <div className="relative flex flex-col h-full">
+            <header className="flex items-center gap-2 p-2 md:hidden">
+              <SidebarTrigger />
+            </header>
             {children}
           </div>
         )}