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/Input.tsx

@@ -0,0 +1,35 @@
+import { cn } from '@/lib/utils'
+
+export interface InputProps extends React.InputHTMLAttributes<HTMLInputElement> {
+  error?: string
+}
+
+export function Input({ className, error, id, ...props }: InputProps) {
+  return (
+    <div>
+      <input
+        id={id}
+        className={cn(
+          'flex h-9 w-full rounded-md border border-border bg-card px-3 py-2 text-sm text-foreground',
+          'placeholder:text-muted-foreground',
+          'focus:border-[rgba(6,182,212,0.3)] focus:outline-none focus:ring-1 focus:ring-primary/20',
+          'disabled:cursor-not-allowed disabled:opacity-50',
+          error && 'border-red-400/50 focus:border-red-400 focus:ring-red-400/20',
+          className
+        )}
+        aria-invalid={error ? true : undefined}
+        aria-describedby={error && id ? `${id}-error` : undefined}
+        {...props}
+      />
+      {error && (
+        <p
+          id={id ? `${id}-error` : undefined}
+          className="mt-1.5 text-xs text-red-400"
+          role="alert"
+        >
+          {error}
+        </p>
+      )}
+    </div>
+  )
+}