Add VNC remote cursor shape sync

Implements RFB's Cursor pseudo-encoding (RFC 6143 SS7.8.2, type -239):
a FramebufferUpdate rectangle carrying a cursor shape instead of
screen content (x/y are the hotspot, not position; width/height are
the cursor image size), decoded into an ARGB32 QImage using the
rectangle's RGB pixel data plus its opacity bitmask, then never
painted into the framebuffer. A 0x0 rectangle means "hide the
cursor" per spec.

VncDisplayWidget gains RdpDisplayWidget's setCursorImage/Hidden/
Default() + applyCursor() shape, reusing its own renderRect()/
effectiveRemoteSize() so cursor scaling works correctly in both the
scale-to-fit and actual-size display modes with no special-casing.
VNC never emits cursorReset() (RFB's Cursor pseudo-encoding has no
"reset to system default" signal, unlike RDP's SetDefault callback) --
setCursorDefault() exists for symmetry but is unused today.

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

src/session_tab.cpp

@@ -274,6 +274,8 @@ SessionTab::SessionTab(const Profile& profile,
                 [this](const QImage& image, const QPoint& hotspot) {
                     if (m_rdpDisplay != nullptr) {
                         m_rdpDisplay->setCursorImage(image, hotspot);
+                    } else if (m_vncDisplay != nullptr) {
+                        m_vncDisplay->setCursorImage(image, hotspot);
                     }
                 },
                 Qt::QueuedConnection);
@@ -283,6 +285,8 @@ SessionTab::SessionTab(const Profile& profile,
                 [this]() {
                     if (m_rdpDisplay != nullptr) {
                         m_rdpDisplay->setCursorHidden();
+                    } else if (m_vncDisplay != nullptr) {
+                        m_vncDisplay->setCursorHidden();
                     }
                 },
                 Qt::QueuedConnection);
@@ -292,6 +296,8 @@ SessionTab::SessionTab(const Profile& profile,
                 [this]() {
                     if (m_rdpDisplay != nullptr) {
                         m_rdpDisplay->setCursorDefault();
+                    } else if (m_vncDisplay != nullptr) {
+                        m_vncDisplay->setCursorDefault();
                     }
                 },
                 Qt::QueuedConnection);