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/window/CollectionDisplayWindow.java

@@ -12,9 +12,9 @@ import com.googlecode.lanterna.input.KeyStroke;
 import com.googlecode.lanterna.input.KeyType;
 import ru.project.collection.MyList;
 import ru.project.student.Student;
-import ru.project.ui.base.BaseWindow;
+import ru.project.ui.base.BaseModalWindow;
 
-public class CollectionDisplayWindow extends BaseWindow {
+public class CollectionDisplayWindow extends BaseModalWindow {
 
   private final MyList<Student> collection;