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_distort_test.cpp

@@ -533,7 +533,7 @@ TEST(CropDistortNode, InputDefinitionsAndDefaults)
 		EXPECT_DOUBLE_EQ(
 			node.get_input_property(side, QStringLiteral("max")).toDouble(), 1.0);
 		EXPECT_EQ(node.get_input_property(side, QStringLiteral("view")).toInt(),
-				  int(olive::FloatSlider::k_percentage));
+				  int(olive::slider::k_percentage));
 	}
 
 	EXPECT_DOUBLE_EQ(
@@ -1521,7 +1521,7 @@ TEST(TileDistortNode, InputDefaults)
 	EXPECT_EQ(node.get_input_property(olive::TileDistortNode::k_scale_input,
 									QStringLiteral("view"))
 				  .toInt(),
-			  int(olive::FloatSlider::k_percentage));
+			  int(olive::slider::k_percentage));
 
 	EXPECT_EQ(node.get_standard_value(olive::TileDistortNode::k_position_input)
 				  .value<QVector2D>(),