feat: add stage 0 foundation

frontend/src/app/layout/AppShell.tsx

@@ -0,0 +1,20 @@
+import type * as React from 'react'
+
+import { Sidebar } from './Sidebar'
+import { Topbar } from './Topbar'
+
+type AppShellProps = {
+  children: React.ReactNode
+}
+
+export function AppShell({ children }: AppShellProps) {
+  return (
+    <div className="min-h-screen bg-background text-foreground">
+      <Sidebar />
+      <div className="min-h-screen lg:pl-64">
+        <Topbar />
+        <main>{children}</main>
+      </div>
+    </div>
+  )
+}