Make the slave limit advisory rather than a refusal

max_slaves records how many contacts a part is expected to carry. It was
enforced as a rule the drawing had to obey, which obstructs the way both
@scorpio810 and @IBSYSLevi described working in #819: draw the schematic
first, choose the physical hardware afterwards. A limit that refuses the
link forces the hardware decision up front, which is exactly what they
said gets in the way.

Two changes, both in the UI rather than in isFull(), which stays the
query it always was:

 - MasterPropertiesWidget::on_link_button_clicked() now says the limit
   is reached and asks whether to link anyway, defaulting to yes,
   instead of refusing outright.

 - LinkSingleElementWidget no longer removes a full master from the
   candidate list. That was the worse half: a master at its limit simply
   was not there, indistinguishable from one that does not exist, with
   nothing to say why. It now stays selectable and the user decides.

PLC masters are deliberately left alone. Their limit is the number of
declared IO slots, which is structural rather than advisory -- a link
past it would have no IO index to map to -- and PlcLinkWidget already
tells the user when it hides one, via m_hidden_masters_label.

Only coils that opt into a limit are affected: max_slaves defaults to
-1, and no project in examples/ sets it.

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

sources/ui/linksingleelementwidget.cpp

@@ -17,7 +17,6 @@
 */
 #include "linksingleelementwidget.h"
 #include "contactgroupselectiondialog.h"
-#include "../qetgraphicsitem/masterelement.h"
 #include "../qetgraphicsitem/conductor.h"
 #include "../diagram.h"
 #include "../diagramposition.h"
@@ -418,11 +417,12 @@ QVector <QPointer<Element>> LinkSingleElementWidget::availableElements()
 				continue;
 			}
 
-			// If the master is full, we'll remove it from the list!
-			MasterElement *master = static_cast<MasterElement*>(elmt);
-			if (master->isFull()) {
-				elmt_vector.removeAt(i);
-			}
+				// A master at its declared limit stays in the list. Removing
+				// it made a full master indistinguishable from one that does
+				// not exist: the candidate simply was not there, with nothing
+				// to say why. The limit is advisory -- see the prompt in
+				// MasterPropertiesWidget::on_link_button_clicked() -- so the
+				// user decides, rather than the list deciding for them.
 		}
 	}
 	return elmt_vector;