#3586911: Fix overlay-link card pattern in course_card SDC
Fixes #3586911
This replaces the invisible overlay anchor with a link on the <h2> course title. A CSS ::after pseudo-element extends the title link's click area to cover the full card, keeping the whole card clickable. The lms-course-card__overlay div and its gradient look visually the same.
This also adds course-name context to every CourseStartLink state using an aria-label on $link['#attributes'], so screen reader users scanning a link list can identify which course each action link belongs to.
I changed overflow: hidden on .lms-course-card to overflow: clip to prevent the card's focus outline from being clipped (a known browser behavior difference between the two values).