feat(webui): add initial webui with websocket chat flow

webui/src/components/EmptyState.tsx

@@ -0,0 +1,26 @@
+import { MessageSquarePlus } from "lucide-react";
+
+import { Button } from "@/components/ui/button";
+
+export function EmptyState({
+  onNewChat,
+}: {
+  onNewChat: () => void;
+}) {
+  return (
+    <div className="flex h-full flex-col items-center justify-center gap-4 text-center">
+      <MessageSquarePlus
+        className="h-10 w-10 text-muted-foreground"
+        aria-hidden
+      />
+      <div className="space-y-1">
+        <p className="text-lg font-medium">No chats yet</p>
+        <p className="max-w-sm text-sm text-muted-foreground">
+          Start a conversation — your sessions are stored locally on the nanobot
+          workspace and stay available across reloads.
+        </p>
+      </div>
+      <Button onClick={onNewChat}>New chat</Button>
+    </div>
+  );
+}