Add session details tooltip on hover to course detail calendar

[abdulateeb]

On the course detail page, the Session Calendar highlights dates that have sessions (teal text + dot indicator), but hovering over those dates provides no information about which session is scheduled. Users must again scroll to the sessions list to find out.

Steps to Reproduce

1. Go to any course detail page that has scheduled sessions
2. Scroll to the Session Calendar section
3. Hover over a teal-highlighted date
4. Notice no tooltip or session info appears

Current Behavior

• Dates with sessions are only highlighted in teal with a small dot
• No tooltip appears on hover - users can't tell what session is on a specific date

Expected Behavior

• Hovering over a calendar date with a session should show a tooltip with session details (title, time)
• Tooltip should have polished UI/UX with smooth transitions, dark mode support, and accessible design