fix: breadcrumb z-index to prevent completion mark overlapping stick…

[MehranKhn]

Problem

When scrolling, the completion checkmark badge on ContentCard was
appearing above the sticky breadcrumb header due to CSS transforms
creating a new stacking context.

Fix

• Increased breadcrumb z-index from z-10 to z-20 in CourseView
• This ensures cards scroll correctly beneath the sticky header

Testing

• Navigate to any course folder
• Scroll down — cards should go below the sticky breadcrumb
• Completion badge should not overlap the header