Add undo/redo support for folio add, delete, and reorder (#575)

Three new QUndoCommand subclasses (AddDiagramCommand, RemoveDiagramCommand,
MoveDiagramCommand) pushed onto the project's existing (already
project-scoped) undo stack, so folio structure edits are undoable
alongside every item-level edit already on that stack.

- QETProject::addDiagram()/detachDiagram() are the shared attach/detach
  primitives: they mutate the diagram list, connect/disconnect the two
  per-diagram signals set up at add time, and emit diagramAdded/
  diagramRemoved. AddDiagramCommand and RemoveDiagramCommand call these
  (via friend access) for both redo and undo, so a removed diagram is
  parked rather than destroyed -- it's only actually deleted if the
  command itself falls out of undo history while still detached.
- ProjectView reacts to diagramRemoved the same way it already reacted to
  diagramAdded (tearing down/rebuilding the tab), so both directions of
  both commands go through the same reactive path every other diagram
  listener (project database, cross-references, generic panel) already
  relies on.
- MoveDiagramCommand wraps a new ProjectView::setDiagramPosition(), which
  performs the tab move and the project's diagramOrderChanged() list
  reorder synchronously in one step, instead of relying on the queued
  tabMoved connection (needed for interactive drag-and-drop) to catch up
  later -- avoiding a second, redundant reorder from that queued call.
- Multi-folio delete and multi-folio move (QETDiagramEditor::removeDiagrams()
  and the moveDiagram*(QList<Diagram*>) batch slots) wrap their per-diagram
  loop in QUndoStack::beginMacro()/endMacro(), so a multi-select action is
  one undo step, matching current UX.
- Softened the delete confirmation's "this change is irreversible" wording
  now that it no longer is.

Verified headlessly (Xvfb + xdotool + scrot): add/undo/redo, delete/undo/
redo (single and multi-select, single undo step for the batch), and
move/undo/redo all behave correctly against a 7-folio project.

sources/undocommand/removediagramcommand.h

@@ -0,0 +1,53 @@
+/*
+	Copyright 2006-2026 The QElectroTech Team
+	This file is part of QElectroTech.
+
+	QElectroTech is free software: you can redistribute it and/or modify
+	it under the terms of the GNU General Public License as published by
+	the Free Software Foundation, either version 2 of the License, or
+	(at your option) any later version.
+
+	QElectroTech is distributed in the hope that it will be useful,
+	but WITHOUT ANY WARRANTY; without even the implied warranty of
+	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+	GNU General Public License for more details.
+
+	You should have received a copy of the GNU General Public License
+	along with QElectroTech.  If not, see <http://www.gnu.org/licenses/>.
+*/
+#ifndef REMOVEDIAGRAMCOMMAND_H
+#define REMOVEDIAGRAMCOMMAND_H
+
+#include <QUndoCommand>
+#include <QPointer>
+
+class QETProject;
+class Diagram;
+
+/**
+	@brief The RemoveDiagramCommand class
+	Undo/redo support for removing a folio (diagram) from a project.
+	redo() detaches the diagram from the project (and, reactively, its
+	ProjectView tab) without destroying it; undo() reinserts it at its
+	original position. The diagram is only actually destroyed when this
+	command itself is destroyed while the diagram is detached (i.e. while
+	the removal is still in effect).
+*/
+class RemoveDiagramCommand : public QUndoCommand
+{
+	public:
+		RemoveDiagramCommand(QETProject *project, Diagram *diagram, QUndoCommand *parent = nullptr);
+		~RemoveDiagramCommand() override;
+
+		void undo() override;
+		void redo() override;
+
+	private:
+		QPointer<QETProject> m_project;
+		Diagram *m_diagram;
+		int m_position;
+		/// true while the diagram is detached from the project (i.e. the removal is in effect)
+		bool m_owns_diagram = false;
+};
+
+#endif // REMOVEDIAGRAMCOMMAND_H