Initial commit: thbd-portal, duplicated from elmi-portal

Rebranded for Thunderbird Portal (THBD): numbering prefix EL->TH, all
"ELMI"/"ЭЛМИ" UI strings genericized, ELMI's real contract templates and
company legal/bank details removed (not applicable to this company), fresh
VAPID keypair and encryption key, fresh single-migration schema history.
Same architecture as elmi-portal: Web Push notifications, unread-chat app
badge, cookie-session auth.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Gh2UXUQUVBWroWEnn1FLFG

src/app/(admin)/presence-heartbeat.tsx

@@ -0,0 +1,25 @@
+"use client";
+
+import { useEffect } from "react";
+
+const INTERVAL_MS = 20_000;
+
+/** Держит lastSeenAt свежим, пока вкладка открыта и видима — "онлайн" на клиенте вычисляется как lastSeenAt моложе ~30с. */
+export function PresenceHeartbeat() {
+  useEffect(() => {
+    function ping() {
+      if (document.visibilityState === "visible") {
+        fetch("/api/presence/heartbeat", { method: "POST" }).catch(() => {});
+      }
+    }
+    ping();
+    const interval = setInterval(ping, INTERVAL_MS);
+    document.addEventListener("visibilitychange", ping);
+    return () => {
+      clearInterval(interval);
+      document.removeEventListener("visibilitychange", ping);
+    };
+  }, []);
+
+  return null;
+}