Rename difficulty to selection_frequency for question display and filters

[DaiIshida4869]

Issue: #63

• Rename "Difficulty" to "Correct Answer Rate" using selection_frequency field
• Fall back to "Difficulty" when selection_frequency is not available

[Copilot]

Pull request overview

Renames “Difficulty” UI/filtering to “Correct Answer Rate” backed by a new selection_frequency field, while attempting to fall back to Difficulty when the new field isn’t present.

Changes:

• Update question filter types and query parameter construction to use min_selection_frequency / max_selection_frequency.
• Add selection_frequency to the Question type and display “Correct Answer Rate” in the list item badge.
• Update filter UI label and edit page UI to show “Correct Answer Rate” with fallback to Difficulty.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/types/QuestionFilters.ts	Renames difficulty filter fields to selection frequency fields.
src/types/Question.ts	Adds selection_frequency to the Question model.
src/hooks/useCourseQuestions.ts	Switches filter query params from difficulty to selection frequency.
src/components/macfast/questions-item/questions-item.tsx	Shows “Correct Answer Rate” badge with fallback to Difficulty.
src/components/macfast/questions-filter.tsx	Renames slider/filter UI to “Correct Answer Rate”.
src/app/courses/[courseCode]/question/[questionId]/edit/tabs/question-tab.tsx	Shows “Correct Answer Rate” on edit page with fallback to Difficulty.

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