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/vnc_display_widget.h

@@ -25,6 +25,19 @@ public:
     void setRemoteDesktopSize(int width, int height);
     void clearFrame();
 
+    // true (default): scale the whole remote screen to fit the widget,
+    // like RdpDisplayWidget. false: render at the remote's actual pixel
+    // size -- meant to be placed inside a QScrollArea, whose scrollbars
+    // then let the user pan around a screen larger than the window
+    // instead of shrinking small text to illegibility. VNC has no
+    // equivalent of RDP's MS-RDPEDISP to request a different resolution
+    // from the guest, so this is the only way to see it at native size.
+    void setScaleToFit(bool scaleToFit);
+    bool scaleToFit() const
+    {
+        return m_scaleToFit;
+    }
+
 signals:
     void keyInput(int key, quint32 nativeScanCode, const QString& text, bool pressed, int modifiers);
     void mouseMoveInput(int x, int y);
@@ -49,12 +62,14 @@ private:
     QImage m_frame;
     QSize m_remoteSize;
     QTimer* m_resizeDebounceTimer;
+    bool m_scaleToFit;
 
     QRectF renderRect() const;
     QPoint mapToRemote(const QPointF& pos) const;
     QSize effectiveRemoteSize() const;
     void emitViewportGeometry();
     void scheduleViewportGeometryEmit();
+    void applySizeConstraint();
 };
 
 #endif