mark state arrays as readonly

src/app/core/models/planned-term.ts

src/app/core/models/year.ts

...p/degree-planner/course-search/course-search.component.ts

src/app/degree-planner/degree-planner.component.ts

...or-later-container/saved-for-later-container.component.ts

...egree-planner/shared/course-item/course-item.component.ts

src/app/degree-planner/shared/utils.ts

src/app/degree-planner/store/effects/plan.effects.ts

src/app/degree-planner/store/reducer.ts

src/app/degree-planner/store/state.ts

...degree-planner/term-container/term-container.component.ts