feat: UI design system primitives, accessibility, and performance improvements

- Add Button component with CVA variants (primary, secondary, destructive, ghost, link)
- Add Input, Textarea, FormField, and Skeleton UI primitives
- Add focus trapping to Modal for WCAG accessibility compliance
- Add prefers-reduced-motion media query for motion-sensitive users
- Add route-level ErrorBoundary wrapping via page() helper in router
- Add route prefetching on sidebar nav hover for instant navigation
- Add PageMeta component with OG/Twitter meta tags (react-helmet-async)
- Add PageMeta to SharedSessionPage and SurveyPage for social sharing
- Replace lodash with custom debounce utility (saves ~71KB bundle)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/layout/NavItem.tsx

@@ -1,6 +1,7 @@
 import { Link, useLocation } from 'react-router-dom'
 import type { LucideIcon } from 'lucide-react'
 import { cn } from '@/lib/utils'
+import { prefetchForRoute } from '@/lib/routePrefetch'
 
 interface NavSubItem {
   href: string
@@ -36,6 +37,7 @@ export function NavItem({ href, icon: Icon, label, badge, matchPaths, collapsed,
     return (
       <Link
         to={href}
+        onMouseEnter={() => prefetchForRoute(href)}
         className={cn(
           'group relative flex items-center justify-center rounded-lg p-2 transition-all duration-120',
           isActive
@@ -61,6 +63,7 @@ export function NavItem({ href, icon: Icon, label, badge, matchPaths, collapsed,
     <div className="group/nav">
       <Link
         to={href}
+        onMouseEnter={() => prefetchForRoute(href)}
         className={cn(
           'group relative flex items-center gap-3 rounded-lg px-3 py-2 text-[0.8125rem] font-medium transition-all duration-120',
           isActive