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.h

@@ -21,7 +21,8 @@ class QTcpSocket;
 // Scope (see plan / issue #3 for the full rationale): standard VNC
 // Authentication (security type 2) and no-auth (type 1) only -- not
 // Apple's Screen Sharing scheme (type 30). Raw + CopyRect encodings only.
-// No dynamic resize, no remote cursor shape sync, no clipboard sync.
+// No dynamic resize, no remote cursor shape sync. Clipboard sync (Latin-1
+// only, per RFB's ServerCutText/ClientCutText) is supported.
 class VncSessionBackend : public SessionBackend
 {
     Q_OBJECT
@@ -52,6 +53,7 @@ public slots:
     void sendMouseMoveEvent(int x, int y) override;
     void sendMouseButtonEvent(int x, int y, int button, bool pressed) override;
     void sendMouseWheelEvent(int x, int y, int deltaX, int deltaY) override;
+    void setClipboardText(const QString& text) override;
 
 private slots:
     void onSocketConnected();
@@ -129,6 +131,7 @@ private:
     void onRectangleFinished();
     void sendPointerEvent();
     void sendWheelClick(quint8 wheelBit);
+    void sendClientCutText(const QString& text);
 };
 
 #endif