feat: three-column mega-menu with Tabler icons and unified nav pills

Redesign the desktop mega-menu as a centered three-column canvas and
unify the navigation interaction language.

- Layout: centered grid (anchor / submenu / device board); below lg the
  side columns drop so tablets get just the submenu column.
- First-level icons mapped by LuCI node name (Tabler set), with a
  default fallback for unmapped sections.
- Submenu fills the middle column top-to-bottom with content-width
  columns, growing rows not width; the panel scrolls only past viewport.
- Unified pill hover/active across top triggers and submenu links in
  both mega-menu and boxed-dropdown modes; drop the old underline.
- Mark the active submenu page so it gets the active pill.
- Device board moves to the right column (cloned per panel), gains an
  i18n "Device" label, uses Tabler icons (router/cpu/package/tag), and
  wraps long values instead of truncating.
- Global prefers-reduced-transparency fallback: drop backdrop blur and
  make the mega-menu background opaque.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: eamonxg