A blockr.core::board_loader() for blockr.core::serve() that picks the
board to build from the request URL. The board named by the URL handle
(board_name / user / version) loads from the session_mgmt_backend
backend; a new handle (minted when the user hits New) yields an empty
board under that fresh id, so saving it forks a distinct record rather than
overwriting the served board; a request without any handle (a cold load)
gets a cleared copy of the served board. The backend is resolved with the
loader's own request (at the GET, before any session) or session (at the WS
connect), so a user-scoped backend such as user_pins_board() resolves
under the visitor's own Posit Connect credentials at both phases; a board
the visitor may not read does not resolve, whatever the user / board_name
in the URL. Pair it with manage_project() when calling
blockr.core::serve():
serve(board, plugins = c(.., manage_project()), loader = rack_loader()).
Value
A blockr.core::board_loader() object.