fix: add active state styling to navbar links

[sh1vam31]

Description

This PR adds active state styling to navbar links to improve navigation clarity and user experience.

Problem

The navbar previously did not visually indicate which page or section the user is currently viewing. All navigation links appeared identical in both active and inactive states, making navigation unclear—especially for first-time users.

Fixes #896

Changes

Desktop Navbar (navbar.tsx)

• Added useRouter hook from Next.js to detect current route
• Implemented isLinkActive() helper function to determine active state for:
  • Hash-based links (e.g., /#about, /#speakers)
  • Route-based links (e.g., /editions, /venue/California)
• Applied visual styling to active links:
  • Bold font weight for active text
  • Visible underline that stays permanent (not just on hover)
• Parent dropdown items (e.g., "Venue") show active state when on child routes
• Submenu items display bold + underline when active

Mobile Navbar (navDrop.tsx)

• Implemented same route detection logic using useRouter
• Applied consistent active styling:
  • Bold text for active links
  • Left border (border-l-4 border-white) for visual indicator on mobile

Visual Changes

Before: All links look the same, no indication of current page
After: Active links display bold text with underline, providing clear visual feedback

Related Issues

Closes #896

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