Implement course enrollment functionality

[deepsonK]

File: frontend/src/pages/Courses.js\n\nIssue: Enroll button only logs, doesn't call API\n\nDifficulty: Medium\n\nWhat to implement:\n1. Implement enrollCourse to call POST /api/courses/:id/enroll\n2. Complete unenrollCourse function\n3. Update UI to show enrollment status\n4. Handle errors