fix: prevent mutation when updating course properties in Calendar's selectDay function

[Abishekcs]

What this PR does

This PR fixes a mutation bug in the selectDay function of the Calendar component. The course object was being directly mutated, which violates React's & Redux state management principles and can lead to unintended side effects.

Cause of the bug

The course object from this.props was being directly modified when updating the day_exceptions and no_day_exceptions properties. Since props are supposed to be immutable and plus this props is from redux slice passed all the way from Course component, this direct mutation could cause unexpected behaviors, such as incorrect renders or state inconsistencies.

Fix of the bug

The fix involves avoiding direct mutation by creating a shallow copy of the course object and updating its properties immutably before passing it to the updateCourse function.

Screenshots

Before:

Before.mp4

After:

After.mp4