[r/boards] initialize pagination

[salmad3]

Context:

Implement pagination for listing boards and threads within the r/boards realm. The existing avl pager package provides a foundation for this functionality, which we'll adapt and extend to meet the needs of boards. Pagination for comments and nested comments being discussed and will follow this update.

Acceptance Criteria:

• Implements pagination for listing boards using the existing avl pager package:

  Example

  boardsPager := pager.NewPager(boardsTree, defaultPageSize)
  page := boardsPager.GetPage(pageNumber)

• Implements pagination for listing threads within a board:

  Example

  threadsPager := pager.NewPager(board.ThreadsTree, defaultPageSize)
  page := threadsPager.GetPageWithSize(pageNumber, pageSize)

• Extends the UI to display paginated results and navigation controls using the Selector() method:

  Example

  paginationUI := page.Selector()

• Uses a hardcoded default page size to start (e.g., 10 items per page)

• Handles invalid page numbers or sizes gracefully by defaulting to the first page

• Ensures compatibility with existing board-related structs

• Includes unit tests for pagination functionality