feat(web): add three-column layout shell with placeholder views

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

web/client/src/App.tsx

@@ -0,0 +1,24 @@
+import { AppLayout } from '@/components/layout/AppLayout';
+import { ChatView } from '@/components/chat/ChatView';
+import { AccountList } from '@/components/accounts/AccountList';
+import { AssetGallery } from '@/components/assets/AssetGallery';
+import { ConfigForm } from '@/components/config/ConfigForm';
+import { useAppStore } from '@/store';
+
+function MainContent() {
+  const view = useAppStore((s) => s.activeView);
+  switch (view) {
+    case 'chat': return <ChatView />;
+    case 'accounts': return <AccountList />;
+    case 'assets': return <AssetGallery />;
+    case 'config': return <ConfigForm />;
+  }
+}
+
+export default function App() {
+  return (
+    <AppLayout>
+      <MainContent />
+    </AppLayout>
+  );
+}