chore(docs): add QA screenshots for #288 student browse & enrollment

GIF recordings:
- student-browse-enroll-flow: public catalog → course detail → browse page with subscription banner → enroll → enrolled course view
- student-my-courses-pricing-payments: my courses (7 enrolled, filters, 2 certs), school pricing page, payment requests history

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: guillermoscript