feat(cms): integrate global layout with CMS and document decision

- Move site-config and navigation to CMS-backed SiteConfigProvider
- Keep static constants as fallback for dev and degraded modes
- Add ADR-0005 recording the global layout CMS strategy

Refs ADR-0005

src/components/layout/mega-dropdown.tsx

@@ -4,7 +4,7 @@ import { useRef, useEffect } from 'react';
 import { ChevronDown, Lock, Sparkles } from 'lucide-react';
 import { StaticLink } from '@/components/ui/static-link';
 import { AnimatePresence, motion } from 'framer-motion';
-import type { MegaDropdownGroup } from '@/lib/constants/navigation';
+import type { MegaDropdownGroup } from '@/lib/site-config';
 
 interface MegaDropdownProps {
   label: string;