feat: add header nav and hamburger menu styling with JS handlers

feat: add header nav and hamburger menu styling with JS handlers

- Add header, nav-link, header-actions component CSS
- Add menu-overlay and menu-nav component CSS
- Update hamburger-menu.html with vanilla JS handlers:
  - openMenu/closeMenu functions
  - ESC key support
  - Click outside to close (backdrop click)
  - Menu link auto-close
  - Scroll lock when menu open
- Replace Alpine @click with id-based event listeners
- Rebuild CSS: main.min.css updated

Co-Authored-By: Claude Haiku 4.5 <redacted>