Add course and exam search filters

[kuroryujinn]

This PR adds client-side search and filter capabilities to the student and teacher dashboards to make course and exam navigation faster and easier.

What changed

• Added a search bar to the student course list.
• Added a search bar and status filter to the student exam list.
• Added a search bar to the teacher courses/batches table.
• Implemented debounced, client-side filtering for already loaded data.

Student side

• Course list can now be filtered by course name or course code.
• Exam list can now be filtered by exam title or batch name.
• Exam list also supports status filtering:
  • All
  • Upcoming
  • Ongoing
  • Ended

Teacher side

• Teacher course/batch list can now be filtered by course name or code.

Notes

• This is frontend-only.
• No backend API changes were required.
• Filtering applies to the currently loaded dataset.

Testing

• Verified the frontend production build passes after the changes.
• Manually checked that search and filter controls update the displayed results as expected.