feat: establish NekoNest Cloud control and relay

app/dashboard/OpenPwaButton.tsx

@@ -0,0 +1,36 @@
+"use client";
+
+import { useState } from "react";
+
+export function OpenPwaButton() {
+  const [pending, setPending] = useState(false);
+  const [error, setError] = useState("");
+
+  async function openPwa() {
+    setPending(true);
+    setError("");
+    try {
+      const response = await fetch("/api/pwa/handoff", { method: "POST" });
+      const payload = await response.json() as {
+        pwa_url?: string;
+        message?: string;
+      };
+      if (!response.ok || !payload.pwa_url) {
+        throw new Error(payload.message || "暂时无法打开手机端");
+      }
+      window.location.assign(payload.pwa_url);
+    } catch (cause) {
+      setError(cause instanceof Error ? cause.message : "暂时无法打开手机端");
+      setPending(false);
+    }
+  }
+
+  return (
+    <span>
+      <button className="button button-primary" type="button" disabled={pending} onClick={openPwa}>
+        {pending ? "正在创建安全入口…" : "打开 NekoNest PWA"}
+      </button>
+      {error && <small className="form-error" role="alert">{error}</small>}
+    </span>
+  );
+}