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/speedduration/speeddurationdialog.cpp

@@ -57,7 +57,7 @@ SpeedDurationDialog::SpeedDurationDialog(const QVector<ClipBlock *> &clips,
 		speed_layout->addWidget(new QLabel(tr("Speed:")), row, 0);
 
 		speed_slider_ = new FloatSlider();
-		speed_slider_->set_display_type(FloatSlider::k_percentage);
+		speed_slider_->set_display_type(slider::k_percentage);
 		connect(speed_slider_, &FloatSlider::value_changed, this,
 				&SpeedDurationDialog::speed_changed);
 		speed_layout->addWidget(speed_slider_, row, 1);
@@ -68,7 +68,7 @@ SpeedDurationDialog::SpeedDurationDialog(const QVector<ClipBlock *> &clips,
 
 		dur_slider_ = new RationalSlider();
 		dur_slider_->set_timebase(timebase);
-		dur_slider_->set_display_type(RationalSlider::k_time);
+		dur_slider_->set_display_type(slider::k_time);
 		connect(dur_slider_, &RationalSlider::value_changed, this,
 				&SpeedDurationDialog::duration_changed);
 		speed_layout->addWidget(dur_slider_, row, 1);