Add VNC clipboard sync

RFB's ServerCutText/ClientCutText messages are much simpler than RDP's
CLIPRDR channel (no format-list/format-request negotiation -- text is
just sent directly in both directions), so ServerCutText now emits
remoteClipboardTextChanged instead of being discarded, and
VncSessionBackend overrides setClipboardText to send ClientCutText
immediately. Extends session_tab.cpp's clipboard-sync gate to cover
VNC alongside RDP; the QClipboard wiring itself was already
protocol-agnostic. Latin-1 only, per RFB's wire format -- no Unicode
clipboard extension is in scope.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/vnc_session_backend.cpp

@@ -263,6 +263,16 @@ void VncSessionBackend::sendMouseWheelEvent(int x, int y, int deltaX, int deltaY
     }
 }
 
+void VncSessionBackend::setClipboardText(const QString& text)
+{
+    if (m_socket->state() != QAbstractSocket::ConnectedState) {
+        return;
+    }
+    // Unlike RDP's CLIPRDR (list -> server request -> response), RFB's
+    // ClientCutText has no negotiation: send it immediately.
+    sendClientCutText(text);
+}
+
 void VncSessionBackend::onSocketConnected()
 {
     emit eventLogged(QStringLiteral("TCP connection established; waiting for VNC handshake."));
@@ -465,6 +475,19 @@ void VncSessionBackend::sendWheelClick(quint8 wheelBit)
     m_socket->write(release);
 }
 
+void VncSessionBackend::sendClientCutText(const QString& text)
+{
+    // Latin-1 only, matching ServerCutText's decode above -- RFB has no
+    // Unicode clipboard extension in scope here.
+    const QByteArray latin1 = text.toLatin1();
+    QByteArray msg;
+    msg.append(kMsgClientCutText);
+    msg.append(3, char(0)); // padding
+    appendU32BE(msg, static_cast<quint32>(latin1.size()));
+    msg.append(latin1);
+    m_socket->write(msg);
+}
+
 void VncSessionBackend::finishHandshakeIntoRunningState()
 {
     emit remoteDesktopSizeChanged(m_framebuffer.width(), m_framebuffer.height());
@@ -895,10 +918,13 @@ void VncSessionBackend::processReceiveBuffer()
             if (!haveBytes(static_cast<int>(m_pendingLength))) {
                 return;
             }
-            // Clipboard sync is out of scope for this pass; consume and
-            // discard so the byte stream stays in sync.
+            // RFB's ServerCutText is Latin-1 only (RFC 6143 SS7.5.4) -- no
+            // Unicode extension is implemented, unlike RDP's CLIPRDR.
+            const QString text =
+                QString::fromLatin1(m_recvBuffer.left(static_cast<int>(m_pendingLength)));
             m_recvBuffer.remove(0, static_cast<int>(m_pendingLength));
             m_rfbState = RfbState::WaitingServerMessageType;
+            emit remoteClipboardTextChanged(text);
             break;
         }
         }