Add live visitor monitoring

public/widget.js

@@ -164,6 +164,11 @@
     });
 
     rememberPage();
+    pingVisitor();
+    setInterval(pingVisitor, 25_000);
+    document.addEventListener("visibilitychange", () => {
+      if (!document.hidden) pingVisitor();
+    });
     if (session.conversationId) {
       loadExistingConversation(messages).finally(() => connectEvents(messages, typing, panel, launcher, settings));
     }
@@ -345,6 +350,10 @@
     };
   }
 
+  function pingVisitor() {
+    postJson(`${apiBase}/api/widget/visitors/ping`, withVisitorContext({})).catch(() => {});
+  }
+
   function showFormError(root, message) {
     const note = root.querySelector(".mf-note");
     note.textContent = message;