Phase 7 of the Bootstrap -> Tailwind migration (symfony#34).

Header / nav are now fully utility-first, with no dedicated stylesheet:
- Express the whole header in Tailwind utilities in _header.html.twig,
  including the blurred ::before overlay (before:* utilities), the open
  transition (@starting-style via the starting: variant +
  transition-discrete), the mobile slide-in menu and social reveal
  (nav:/max-nav:, group-[.open]:), and the light/white theming through
  conditional classes (text-white / dark:invert / before:bg-*) instead of
  CSS custom properties.
- Extract the repeated markup into Twig components to avoid duplicating
  long class lists: AppNav:Item (nav link, optional badge + responsive
  hide), AppNav:Badge, AppNav:IconLink (social icon).
- Scroll-lock now toggles the Tailwind overflow-y-hidden class on body
  from the ux-header controller (was a custom .locked rule).
- The ThemeSwitcher icon inherits the header color (text-current) so it
  stays white on the white package header.

Hero:
- Convert .hero/.hero-background/.hero-sub-text to utilities across the
  ~14 templates that used them (pt-20/pt-16, w-1/2 + responsive widths,
  text-balance). Drop the unused .hero-terminal rule.

Cleanup:
- Remove the three section imports from app.scss and delete _header.scss,
  _nav.scss, _hero.scss. No section stylesheet remains.
- Update IconsTest to look for the Icons link in the header element
  (the .AppNav_menu class no longer exists).

Verified light/dark on desktop, white package header, and mobile
(slide-in menu, blur overlay with starting-style fade, social reveal,
body scroll lock).