Add VNC support (Milestone 6, issue #3)

Implements RFB (RFC 6143) directly against QTcpSocket. No permissively
licensed VNC client library exists to vendor the way FreeRDP was for RDP:
LibVNCClient is GPLv2, gtk-vnc is LGPL but GTK-tied, and vendoring either
would force a licensing decision on the whole (MIT) project. This is
from-scratch protocol code instead, threaded like SshSessionBackend (a
QObject on its own QThread driven by Qt's own async socket signals)
rather than RdpSessionBackend's manual worker-thread/blocking-loop
pattern, since QTcpSocket is already async.

Scope, matching SessionTab's existing SSH/RDP dispatch pattern
(session_backend_factory.cpp, session_tab.cpp's widget construction and
signal wiring) and VncDisplayWidget mirroring RdpDisplayWidget's
scale-to-fit rendering:
- Protocol handshake: RFB 3.3/3.7/3.8 negotiated explicitly (the
  SecurityResult message only exists in 3.8; pre-3.8 servers signal auth
  failure by closing the socket, which the disconnect handler accounts
  for)
- VNC Authentication (DES challenge-response, via OpenSSL's classic DES
  API) and no-auth security types
- Raw + CopyRect framebuffer decoding into a persistent QImage, requesting
  a fixed 32bpp format whose byte layout matches QImage::Format_RGB32
  directly (same zero-conversion trick RdpSessionBackend uses for
  FreeRDP's GDI buffer)
- Keyboard (Qt key -> X11 keysym, including the Unicode-beyond-Latin-1
  keysym convention) and mouse/wheel input forwarding

Explicit non-goals for this pass (see docs/PROGRESS.md for the full
list): Apple's Screen Sharing auth (so this can't yet reach macOS's
built-in VNC server), compression encodings beyond Raw/CopyRect, dynamic
resize, remote cursor shape sync, clipboard sync.

19 unit tests (tests/test_vnc_session_backend.cpp): pure-function
coverage (DES key prep verified against an independently documented test
vector for password "COW", X11 keysym mapping, socket-error mapping) plus
state-machine coverage against a scripted in-process fake RFB server
covering all three protocol-version handshake shapes, auth success/
failure, unsupported security types, and pixel-accurate Raw decoding.
That harness caught a real re-entrancy bug: QAbstractSocket::abort()
synchronously re-emits disconnected() before returning, so
failConnection() calling it was silently letting a second, generic
disconnected-socket handler overwrite an already-correct, specific error
message.

Also verified live against a real, independently implemented VNC server
(TightVNC on Windows): connect with VNC Authentication, correct
framebuffer dimensions and pixel data, clean disconnect, reconnect.

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

src/session_tab.cpp

@@ -3,6 +3,7 @@
 #include "rdp_display_widget.h"
 #include "session_backend_factory.h"
 #include "terminal_view.h"
+#include "vnc_display_widget.h"
 
 #include <KodoTerm/KodoTerm.hpp>
 
@@ -79,6 +80,7 @@ SessionTab::SessionTab(const Profile& profile,
                                 == 0),
       m_sshTerminal(nullptr),
       m_rdpDisplay(nullptr),
+      m_vncDisplay(nullptr),
       m_terminalOutput(nullptr),
       m_eventLog(nullptr),
       m_toggleEventsButton(nullptr),
@@ -238,6 +240,8 @@ SessionTab::SessionTab(const Profile& profile,
                 [this](const QImage& frame) {
                     if (m_rdpDisplay != nullptr) {
                         m_rdpDisplay->setFrame(frame);
+                    } else if (m_vncDisplay != nullptr) {
+                        m_vncDisplay->setFrame(frame);
                     }
                 },
                 Qt::QueuedConnection);
@@ -247,6 +251,8 @@ SessionTab::SessionTab(const Profile& profile,
                 [this](int width, int height) {
                     if (m_rdpDisplay != nullptr) {
                         m_rdpDisplay->setRemoteDesktopSize(width, height);
+                    } else if (m_vncDisplay != nullptr) {
+                        m_vncDisplay->setRemoteDesktopSize(width, height);
                     }
                 },
                 Qt::QueuedConnection);
@@ -405,6 +411,12 @@ void SessionTab::clearTerminal()
     if (m_rdpDisplay != nullptr) {
         m_rdpDisplay->clearFrame();
         m_rdpDisplay->setFocus();
+        return;
+    }
+
+    if (m_vncDisplay != nullptr) {
+        m_vncDisplay->clearFrame();
+        m_vncDisplay->setFocus();
     }
 }
 
@@ -717,6 +729,9 @@ void SessionTab::setupUi()
     } else if (m_profile.protocol.compare(QStringLiteral("RDP"), Qt::CaseInsensitive) == 0) {
         m_rdpDisplay = new RdpDisplayWidget(this);
         rootLayout->addWidget(m_rdpDisplay, 1);
+    } else if (m_profile.protocol.compare(QStringLiteral("VNC"), Qt::CaseInsensitive) == 0) {
+        m_vncDisplay = new VncDisplayWidget(this);
+        rootLayout->addWidget(m_vncDisplay, 1);
     } else {
         m_terminalOutput = new TerminalView(this);
         QFont fallbackFont = defaultTerminalFont();
@@ -726,13 +741,7 @@ void SessionTab::setupUi()
         m_terminalOutput->setFont(fallbackFont);
         m_terminalOutput->setMinimumHeight(260);
         m_terminalOutput->setReadOnly(true);
-        if (m_profile.protocol.compare(QStringLiteral("VNC"), Qt::CaseInsensitive) == 0) {
-            m_terminalOutput->setPlaceholderText(
-                QStringLiteral("Embedded VNC session output appears here when the backend is available."));
-        } else {
-            m_terminalOutput->setPlaceholderText(
-                QStringLiteral("Session output appears here."));
-        }
+        m_terminalOutput->setPlaceholderText(QStringLiteral("Session output appears here."));
         rootLayout->addWidget(m_terminalOutput, 1);
     }
 
@@ -891,6 +900,37 @@ void SessionTab::setupUi()
                 [this](int x, int y, int deltaX, int deltaY) {
                     emit requestMouseWheelEvent(x, y, deltaX, deltaY);
                 });
+    } else if (m_vncDisplay != nullptr) {
+        connect(m_vncDisplay,
+                &VncDisplayWidget::viewportSizeChanged,
+                this,
+                [this](int width, int height) { emit requestTerminalSize(width, height); });
+        connect(m_vncDisplay,
+                &VncDisplayWidget::displayScaleChanged,
+                this,
+                [this](qreal ratio) { emit requestDisplayScale(ratio); });
+        connect(m_vncDisplay,
+                &VncDisplayWidget::keyInput,
+                this,
+                [this](int key, quint32 nativeScanCode, const QString& text, bool pressed, int modifiers) {
+                    emit requestKeyEvent(key, nativeScanCode, text, pressed, modifiers);
+                });
+        connect(m_vncDisplay,
+                &VncDisplayWidget::mouseMoveInput,
+                this,
+                [this](int x, int y) { emit requestMouseMoveEvent(x, y); });
+        connect(m_vncDisplay,
+                &VncDisplayWidget::mouseButtonInput,
+                this,
+                [this](int x, int y, int button, bool pressed) {
+                    emit requestMouseButtonEvent(x, y, button, pressed);
+                });
+        connect(m_vncDisplay,
+                &VncDisplayWidget::mouseWheelInput,
+                this,
+                [this](int x, int y, int deltaX, int deltaY) {
+                    emit requestMouseWheelEvent(x, y, deltaX, deltaY);
+                });
     }
 }
 
@@ -1110,6 +1150,14 @@ void SessionTab::refreshActionButtons()
         if (isConnected) {
             m_rdpDisplay->setFocus();
         }
+        return;
+    }
+
+    if (m_vncDisplay != nullptr) {
+        m_vncDisplay->setEnabled(isConnected);
+        if (isConnected) {
+            m_vncDisplay->setFocus();
+        }
     }
 }