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/common/crashpadinterface.cpp

@@ -27,7 +27,6 @@
 #include <QDateTime>
 #include <QDebug>
 #include <QDir>
-#include <QMessageBox>
 #include <QProcess>
 
 #include "crashpadutils.h"

app/common/dropworkflowbehavior.h

@@ -0,0 +1,43 @@
+/***
+
+  Oak - Non-Linear Video Editor
+  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/>.
+
+***/
+
+#ifndef OAK_DROPWORKFLOWBEHAVIOR_H
+#define OAK_DROPWORKFLOWBEHAVIOR_H
+
+namespace olive
+{
+
+/**
+ * @brief Behavior when media is dropped onto a timeline without a sequence
+ *
+ * Shared by the config defaults (engine layer) and the timeline import
+ * tool (UI layer). Enumerator order matches the previous
+ * ImportTool::DropWithoutSequenceBehavior.
+ */
+enum DropWithoutSequenceBehavior {
+	k_dws_ask,
+	k_dws_auto,
+	k_dws_manual,
+	k_dws_disable
+};
+
+}
+
+#endif // OAK_DROPWORKFLOWBEHAVIOR_H