gui: guard Gui API against null main_window in headless/web mode

In -web/headless mode the static main_window is null while Gui::enabled()
is true (the web HeadlessViewer is installed). Public Gui methods
dereferenced main_window unconditionally, so any call reaching them from
the web render thread or from a module's debug-graphics renderer crashed
(issue #10576, e.g. HeatMapRenderer::drawObjects ->
Renderer::checkDisplayControl -> Gui::checkDisplayControlsVisible).

- Add an early `if (!hasUI())` guard to the affected Gui methods: no-op
  for void methods, safe default (0/""/false/empty static) otherwise.
- Route the display-control accessors to the HeadlessViewer when the Qt
  widget is absent; extend HeadlessViewer with default-safe virtuals.
- Replace the false-positive `enabled()` guards (true in web) with
  `hasUI()` in saveImage/saveClockTreeImage/saveHistogramImage/
  showWorstTimingPath/clearTimingPath/selectClockviewerClock/selectHelp/
  selectChart/gifStart/setLogger.
- Guard the static findWidget() helper at its root.

The guards only fire when main_window is null, so interactive -gui
behavior is unchanged.

Signed-off-by: Jorge Ferreira <jorge.ferreira@precisioninno.com>

Author: jorge-ferreira-pii

src/gui/include/gui/gui.h

@@ -741,6 +741,24 @@ class HeadlessViewer
   // True while pause() is blocking.  Consumers (like the web tile renderer)
   // can use this to gate unsafe cross-thread reads of renderer state.
   virtual bool isPaused() const = 0;
+
+  // Display-control accessors consulted by Gui::*DisplayControls* when the Qt
+  // GUI (and its DisplayControls widget) is absent.  Default implementations
+  // assume "everything visible, nothing selectable" so headless renderers draw
+  // by default without a Qt widget.  Viewers that track their own visibility
+  // model (e.g. the web viewer) may override these.
+  virtual bool checkDisplayControlVisible(const std::string& name)
+  {
+    return true;
+  }
+  virtual bool checkDisplayControlSelectable(const std::string& name)
+  {
+    return false;
+  }
+  virtual void setDisplayControlVisible(const std::string& name, bool value) {}
+  virtual void setDisplayControlSelectable(const std::string& name, bool value)
+  {
+  }
 };
 
 // This is the API for the rest of the program to interact with the