Migrate "Study Levels" & "Groups" (Hierarchical CRUD)

[Ryan2486]

Model: Define Level interface with a nested Group[] array.

• Service: Create LevelService to fetch all levels and GroupService to handle groups via levelId.
• UI - Master: Develop LevelListComponent (Table) showing the list of levels.
• UI - Detail: Create a "Level Detail" view where user can see and manage (Add/Edit/Delete) the Groups belonging to that specific level.
• Forms: Implement Reactive Forms for both Level (Simple) and Group (Nested).
• API: Integrate with existing /levels and /levels/:id/groups endpoints.