feat(collection): implement custom collection creation flow

- rename window base class usage to BaseModalWindow and update modal method calls
- add persistent CustomDataSettingsWindow instance to CreateCollectionMenu
- prepare custom data collection dynamically before showing the settings window
- apply custom data to AppState without immediately replacing mainCollection
- create mainCollection from prepared custom student data
- replace student preview message with collection preview window
- update input validation and StudentBuilder state handling
- rename collection-related methods and variables for clearer semantics

app/src/main/java/ru/project/ui/menu/FillMethodSelectionWindow.java

@@ -4,9 +4,9 @@ import com.googlecode.lanterna.gui2.ActionListBox;
 import com.googlecode.lanterna.gui2.WindowBasedTextGUI;
 import java.util.function.Consumer;
 import ru.project.model.FillMethod;
-import ru.project.ui.base.BaseWindow;
+import ru.project.ui.base.BaseModalWindow;
 
-public class FillMethodSelectionWindow extends BaseWindow {
+public class FillMethodSelectionWindow extends BaseModalWindow {
 
   private final Consumer<FillMethod> onFillMethodSelected;