Regenerate conductor uuids when a folio is duplicated

ElementsPanelWidget::duplicateDiagram() round-trips the folio through XML
and then gives the copied *elements* fresh uuids, because element.uuid is
the primary key of the project database and a duplicate silently fails to
insert. Conductors now have the same problem and needed the same loop:
conductor.uuid is likewise a primary key, its insert is a plain INSERT
rather than INSERT OR IGNORE, and a failure only reaches qDebug(). Without
this, every wire on a duplicated folio is missing from the wiring list and
from the per-element wire count, with nothing shown to the user.

Verified against the real schema: inserting the same conductor uuid for a
second folio fails with "UNIQUE constraint failed: conductor.uuid", leaving
one row where two were expected.

Also harden the uuid read in Conductor::fromXml(). The default argument of
QDomElement::attribute() is evaluated whether or not the attribute exists,
so a uuid was minted for every conductor on every load and thrown away; and
the default only applies when the attribute is *absent*, so a present but
empty or malformed uuid="" parsed to a null QUuid rather than a fresh one --
and null uuids collide with each other exactly as duplicates do.

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

sources/elementspanelwidget.cpp

@@ -17,6 +17,7 @@
 */
 #include "elementspanelwidget.h"
 #include "diagram.h"
+#include "qetgraphicsitem/conductor.h"
 #include "editor/ui/qetelementeditor.h"
 #include "elementscategoryeditor.h"
 #include "qetapp.h"
@@ -652,6 +653,13 @@ void ElementsPanelWidget::duplicateDiagram()
 				elmt->newUuid();
 				new_diagram->restoreText(elmt);
 			}
+			else if (Conductor *cond = dynamic_cast<Conductor *>(item)) {
+				// Same reasoning for conductors: conductor.uuid is the PRIMARY
+				// KEY of the conductor table, and its insert is a plain INSERT,
+				// so a duplicated uuid fails and the wire silently disappears
+				// from the wiring list and the per-element wire count.
+				cond->newUuid();
+			}
 		}
 	}