Add mobile bottom navigation with local Lucide icons and improved accessibility

- Add local Lucide icon system (5 optimized icons)
- Create Apple-style bottom navigation bar (fixed at bottom on mobile)
- Add language toggle (replaces dropdown selector)
- Improve theme support with proper light/dark mode colors
- Add comprehensive ARIA labels and semantic HTML
- Hide main menu on mobile (≤666px) when bottom nav is active
- Add hreflang attributes for language switching

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

assets/css/_colors.css

@@ -97,7 +97,7 @@
   --color-backdrop:               light-dark(var(--color-black-60), var(--color-black-80));
 
   /* Mobil menu colours. */
-  --color-mobile-menu:            light-dark(var(--color-brand-400), var(--color-brand-400));
+  --color-mobile-menu:            light-dark(var(--color-white), var(--color-brand-400));
   --color-mobile-menu-cover:      light-dark(var(--color-black-20), var(--color-black-20));
 
   /* Form colours. */