Add VNC-only scale-to-fit vs actual-size display toggle

VNC has no equivalent of RDP's MS-RDPEDISP to request a different
resolution from the guest, so a high-resolution remote desktop
previously always got shrunk to fit the window, making text
illegible. Adds a per-tab "Display Mode" choice (tab-bar right-click)
between the existing scale-to-fit behavior and a new actual-size mode
that renders the framebuffer at its native pixel size inside a
QScrollArea. Reuses VncDisplayWidget's existing scale-to-fit render
math unchanged -- it degenerates to an exact 1:1 mapping once the
widget is fixed to the remote's own size. Persisted like the terminal
theme preference.

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

src/session_tab.h

@@ -23,6 +23,7 @@ class QLineEdit;
 class QComboBox;
 class QLabel;
 class QPushButton;
+class QScrollArea;
 class KodoTerm;
 
 struct SessionUiPreferences
@@ -30,6 +31,7 @@ struct SessionUiPreferences
     QString terminalThemeName = QStringLiteral("Dark");
     bool eventsPanelExpanded = false;
     int terminalFontPointSize = 0;
+    bool vncScaleToFit = true;
 };
 
 class SessionTab : public QWidget
@@ -62,6 +64,9 @@ public:
     void clearEvents();
     void copyEvents() const;
     void exportEventsToFile();
+    bool supportsVncScaleToggle() const;
+    void setVncScaleToFit(bool scaleToFit);
+    bool vncScaleToFit() const;
 
 signals:
     void tabTitleChanged(const QString& title);
@@ -69,6 +74,7 @@ signals:
     void terminalThemeChanged(const QString& themeName);
     void terminalFontSizeChanged(int pointSize);
     void eventsPanelVisibilityChanged(bool expanded);
+    void vncScaleModeChanged(bool scaleToFit);
     void requestConnect(const SessionConnectOptions& options);
     void requestDisconnect();
     void requestReconnect(const SessionConnectOptions& options);
@@ -111,6 +117,7 @@ private:
     KodoTerm* m_sshTerminal;
     RdpDisplayWidget* m_rdpDisplay;
     VncDisplayWidget* m_vncDisplay;
+    QScrollArea* m_vncScrollArea;
     TerminalView* m_terminalOutput;
     QPlainTextEdit* m_eventLog;
     QToolButton* m_toggleEventsButton;