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)

tests/gtest/node_audio_test.cpp

@@ -227,7 +227,7 @@ TEST(PanNode, InputDefaults)
 	EXPECT_EQ(int(pan.get_input_property(olive::PanNode::k_panning_input,
 									   QStringLiteral("view"))
 					  .toInt()),
-			  int(olive::FloatSlider::k_percentage));
+			  int(olive::slider::k_percentage));
 }
 
 TEST(PanNode, RetranslateSetsInputNames)
@@ -489,7 +489,7 @@ TEST(VolumeNode, InputDefaults)
 	EXPECT_EQ(int(volume.get_input_property(olive::VolumeNode::k_volume_input,
 										  QStringLiteral("view"))
 					  .toInt()),
-			  int(olive::FloatSlider::k_decibel));
+			  int(olive::slider::k_decibel));
 }
 
 TEST(VolumeNode, RetranslateSetsInputNames)