stcp: ed25519 pubkey exchange in handshake, fix instance destroy segfault, fix chat_tcp_test

- u_async.c: process immediate_queue BEFORE epoll_wait in uasync_poll
- stcp_link.c: defensive NULL etcp_conn/etcp_link in stcp_link_close
- utun_instance.c: move stcp_server_list_destroy_all to Phase J (before ETCP),
  add deferred drain in both Phase J and L
- stcp.h/c: STCP_HS_ENC_CLIENT 6→38, STCP_HS_ENC_SERVER 7→39,
  exchange ed25519_pubkey in both handshake directions
- etcp_connections.c: copy peer_ed25519 from STCP to ETCP_CONN in
  etcp_link_enter_ready_tcp; guard link->etcp in tcp_link_close_cb
- chat_sync.c: fix lk->conn NULL dereference for TCP links
- test_stcp.c: update stcp_server_create/stcp_client_connect callsites
- tools/chat_tcp_test: change transport udp→tcp

tools/chatgui/src/invitedialog.cpp

@@ -0,0 +1,50 @@
+#include "invitedialog.h"
+#include "qrcode_utils.h"
+#include <QVBoxLayout>
+#include <QHBoxLayout>
+#include <QLineEdit>
+#include <QPushButton>
+#include <QLabel>
+#include <QClipboard>
+#include <QApplication>
+
+ShareInviteDialog::ShareInviteDialog(const QString& link, QWidget* parent)
+    : QDialog(parent), m_link(link)
+{
+    setWindowTitle(QString::fromUtf8("Пригласительная ссылка"));
+    setMinimumWidth(420);
+    setModal(true);
+
+    auto *layout = new QVBoxLayout(this);
+    layout->setSpacing(12);
+
+    auto *linkEdit = new QLineEdit(m_link, this);
+    linkEdit->setReadOnly(true);
+    linkEdit->setStyleSheet("padding: 6px; font-family: monospace;");
+    layout->addWidget(linkEdit);
+
+    auto *btnLayout = new QHBoxLayout();
+    auto *copyBtn = new QPushButton(QString::fromUtf8("Копировать"), this);
+    connect(copyBtn, &QPushButton::clicked, this, [this, linkEdit]() {
+        QClipboard* cb = QApplication::clipboard();
+        cb->setText(m_link, QClipboard::Clipboard);
+        if (cb->supportsSelection())
+            cb->setText(m_link, QClipboard::Selection);
+        linkEdit->setFocus();
+    });
+    btnLayout->addWidget(copyBtn);
+    btnLayout->addStretch();
+    layout->addLayout(btnLayout);
+
+    auto *qrLabel = new QLabel(this);
+    QImage qrImage = generateQRCode(m_link);
+    if (!qrImage.isNull())
+        qrLabel->setPixmap(QPixmap::fromImage(qrImage).scaled(
+            180, 180, Qt::KeepAspectRatio, Qt::SmoothTransformation));
+    qrLabel->setAlignment(Qt::AlignCenter);
+    layout->addWidget(qrLabel, 0, Qt::AlignCenter);
+
+    auto *closeBtn = new QPushButton("OK", this);
+    connect(closeBtn, &QPushButton::clicked, this, &QDialog::accept);
+    layout->addWidget(closeBtn, 0, Qt::AlignRight);
+}