refactor: cut engine-to-UI include violations ahead of the liboakengine split

- slider DisplayType enums sink to node/sliderdisplaytype.h (canonical
  engine home); FloatSlider/RationalSlider alias them for compatibility
- DropWithoutSequenceBehavior enum sinks to common/dropworkflowbehavior.h
- Config errors now go through a registered ErrorHandler hook instead of
  QMessageBox with a MainWindow parent; the style default no longer
  depends on the UI style manager
- MainWindowLayoutInfo moves to node/project/serializer/ and its panel
  dependency is reduced to a plain std::map alias (PanelLayoutInfo),
  breaking the engine -> PanelWidget -> KDDockWidgets chain
- ProjectImportErrorDialog moves from task/ to dialog/projectimport/
- remove confirmed-redundant UI includes and give project.h/import.h/
  project.cpp the direct includes they were borrowing transitively
- project.h includes folder/sequence headers directly (it used both
  types in its own API all along)

app/dialog/projectimport/CMakeLists.txt

@@ -0,0 +1,23 @@
+# Olive - Non-Linear Video Editor
+# Copyright (C) 2022 Olive Team
+# Modifications Copyright (C) 2026 Oak Team
+#
+# This program 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 3 of the License, or
+# (at your option) any later version.
+#
+# This program 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 this program.  If not, see <http://www.gnu.org/licenses/>.
+
+set(OLIVE_SOURCES
+        ${OLIVE_SOURCES}
+        dialog/projectimport/projectimporterrordialog.cpp
+        dialog/projectimport/projectimporterrordialog.h
+        PARENT_SCOPE
+)

app/dialog/projectimport/projectimporterrordialog.cpp

@@ -0,0 +1,58 @@
+/***
+
+  Olive - Non-Linear Video Editor
+  Copyright (C) 2022 Olive Team
+  Modifications Copyright (C) 2025 mikesolar
+
+  This program 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 3 of the License, or
+  (at your option) any later version.
+
+  This program 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 this program.  If not, see <http://www.gnu.org/licenses/>.
+
+***/
+
+#include "projectimporterrordialog.h"
+
+#include <QDialogButtonBox>
+#include <QLabel>
+#include <QListWidget>
+#include <QVBoxLayout>
+
+namespace olive
+{
+
+ProjectImportErrorDialog::ProjectImportErrorDialog(const QStringList &filenames,
+												   QWidget *parent)
+	: QDialog(parent)
+{
+	QVBoxLayout *layout = new QVBoxLayout(this);
+
+	setWindowTitle(tr("Import Error"));
+
+	layout->addWidget(new QLabel(tr(
+		"The following files failed to import. Oak Video Editor likely does not "
+		"support their formats.")));
+
+	QListWidget *list_widget = new QListWidget();
+	foreach (const QString &s, filenames) {
+		list_widget->addItem(s);
+	}
+	layout->addWidget(list_widget);
+
+	QDialogButtonBox *buttons = new QDialogButtonBox();
+	buttons->setStandardButtons(QDialogButtonBox::Ok);
+	buttons->setCenterButtons(true);
+	connect(buttons, &QDialogButtonBox::accepted, this,
+			&ProjectImportErrorDialog::accept);
+	layout->addWidget(buttons);
+}
+
+}

app/dialog/projectimport/projectimporterrordialog.h

@@ -0,0 +1,41 @@
+/***
+
+  Olive - Non-Linear Video Editor
+  Copyright (C) 2022 Olive Team
+  Modifications Copyright (C) 2025 mikesolar
+
+  This program 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 3 of the License, or
+  (at your option) any later version.
+
+  This program 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 this program.  If not, see <http://www.gnu.org/licenses/>.
+
+***/
+
+#ifndef OAK_PROJECTIMPORTERRORDIALOG_H
+#define OAK_PROJECTIMPORTERRORDIALOG_H
+
+#include <QDialog>
+
+#include "common/define.h"
+
+namespace olive
+{
+
+class ProjectImportErrorDialog : public QDialog {
+	Q_OBJECT
+public:
+	ProjectImportErrorDialog(const QStringList &filenames,
+							 QWidget *parent = nullptr);
+};
+
+}
+
+#endif // OAK_PROJECTIMPORTERRORDIALOG_H