Fix: Ensure CheckCircle2 respects navbar z-index on scroll

[karan-vaishnav]

PR Fixes:

• 1 Adjusted the z-index of the navbar in courseView.tsx to 20 to ensure it stays above all card components, including the CheckCircle2 icon.
• 2 Fixed stacking context issues causing the CheckCircle2 icon to overlap the navbar when scrolling.

Resolves #1716

Checklist before requesting a review

• [x] I have performed a self-review of my code