ENH: Add sticky lecture series title to navbar on scroll

Feature Request

Add a dynamic lecture series title to the top navbar that appears when the user scrolls past the page header, maintaining context for readers navigating long pages.

Behavior

• Trigger: When the lecture series heading in the page header scrolls out of view
• Display: Show the series title in the center of the top navbar
• Animation: Fade in/out transition for smooth appearance
• Responsive: Only active on desktop/tablet screens (disable on mobile due to space constraints)

Implementation Details

Technical Approach

1. JavaScript: Use Intersection Observer API to detect when page header is visible/hidden
2. Dynamic Content: Inject/show series title in navbar based on scroll position
3. CSS: Add fade transitions and center positioning for navbar title
4. Responsive: Use media queries to disable on mobile screens

Navbar Integration

• Placement: Center of navbar
• Styling: Match existing navbar style and typography
• Space Management: Ensure it doesn't conflict with logo, search, or action buttons

Files to Modify

• src/quantecon_book_theme/assets/scripts/index.js - Add scroll detection logic
• src/quantecon_book_theme/theme/quantecon_book_theme/layout.html - Add navbar title container
• src/quantecon_book_theme/assets/styles/index.scss - Add styling for dynamic title

Benefits

• Improved navigation context for long-form content
• Better user experience when scrolling through lectures
• Maintains visual connection to lecture series while reading

Related

• Part of ongoing header improvements from #328
• Complements git metadata features from #331

Read src/quantecon_book_theme/assets/scripts/index.js, src/quantecon_book_theme/theme/quantecon_book_theme/layout.html, and src/quantecon_book_theme/assets/styles/index.scss. Start by tracing the existing navbar and page-header structure, then verify the title appears only after the heading leaves view, fades smoothly, remains centered without conflicts, and is disabled on mobile.