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/ui/FormField.tsx

@@ -0,0 +1,24 @@
+import type { ReactNode } from 'react'
+
+interface FormFieldProps {
+  label: string
+  htmlFor?: string
+  required?: boolean
+  hint?: string
+  children: ReactNode
+}
+
+export function FormField({ label, htmlFor, required, hint, children }: FormFieldProps) {
+  return (
+    <div className="space-y-1.5">
+      <label htmlFor={htmlFor} className="text-sm font-medium text-foreground">
+        {label}
+        {required && <span className="ml-0.5 text-red-400">*</span>}
+      </label>
+      {children}
+      {hint && (
+        <p className="text-xs text-muted-foreground">{hint}</p>
+      )}
+    </div>
+  )
+}