Convertir materias optativas en nodos

> Pedido: que los créditos por fuera del plan aparezcan con el nombre que uno le pone pero en una cajita al igual que las materias obligatorias y electivas del plan

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No files, tests, or entry points are named in the issue. Start by locating the code that renders mandatory and elective subjects as boxes, then trace how credits outside the plan are represented. Done means a user-provided name for each optional subject is displayed in a matching node, consistently with the existing subject types.

Written by the indexing model from the issue text.