feat: add DisplayOptionsPreview component with debouncing

Add client component that fetches and displays live preview of display
options with 300ms debouncing to reduce server load.

- Uses React Query for caching and loading states
- Debounces checkbox changes to avoid excessive API calls
- Shows "Generating preview..." during fetch
- Consistent 200px minimum height to prevent layout shift
- Renders SVG using dangerouslySetInnerHTML

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

apps/web/src/app/create/worksheets/addition/components/DisplayOptionsPreview.tsx

@@ -0,0 +1,136 @@
+'use client'
+
+import { useQuery } from '@tanstack/react-query'
+import { useEffect, useState } from 'react'
+import { css } from '../../../../../../styled-system/css'
+import type { WorksheetFormState } from '../types'
+
+interface DisplayOptionsPreviewProps {
+  formState: WorksheetFormState
+}
+
+async function fetchExample(options: {
+  showCarryBoxes: boolean
+  showAnswerBoxes: boolean
+  showPlaceValueColors: boolean
+  showProblemNumbers: boolean
+  showCellBorder: boolean
+  showTenFrames: boolean
+  showTenFramesForAll: boolean
+}): Promise<string> {
+  const response = await fetch('/api/create/worksheets/addition/example', {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify({
+      ...options,
+      fontSize: 16,
+    }),
+  })
+
+  if (!response.ok) {
+    throw new Error('Failed to fetch example')
+  }
+
+  const data = await response.json()
+  return data.svg
+}
+
+export function DisplayOptionsPreview({ formState }: DisplayOptionsPreviewProps) {
+  // Debounce the display options to avoid hammering the server
+  const [debouncedOptions, setDebouncedOptions] = useState({
+    showCarryBoxes: formState.showCarryBoxes ?? true,
+    showAnswerBoxes: formState.showAnswerBoxes ?? true,
+    showPlaceValueColors: formState.showPlaceValueColors ?? true,
+    showProblemNumbers: formState.showProblemNumbers ?? true,
+    showCellBorder: formState.showCellBorder ?? true,
+    showTenFrames: formState.showTenFrames ?? false,
+    showTenFramesForAll: formState.showTenFramesForAll ?? false,
+  })
+
+  useEffect(() => {
+    const timer = setTimeout(() => {
+      setDebouncedOptions({
+        showCarryBoxes: formState.showCarryBoxes ?? true,
+        showAnswerBoxes: formState.showAnswerBoxes ?? true,
+        showPlaceValueColors: formState.showPlaceValueColors ?? true,
+        showProblemNumbers: formState.showProblemNumbers ?? true,
+        showCellBorder: formState.showCellBorder ?? true,
+        showTenFrames: formState.showTenFrames ?? false,
+        showTenFramesForAll: formState.showTenFramesForAll ?? false,
+      })
+    }, 300) // 300ms debounce
+
+    return () => clearTimeout(timer)
+  }, [
+    formState.showCarryBoxes,
+    formState.showAnswerBoxes,
+    formState.showPlaceValueColors,
+    formState.showProblemNumbers,
+    formState.showCellBorder,
+    formState.showTenFrames,
+    formState.showTenFramesForAll,
+  ])
+
+  const { data: svg, isLoading } = useQuery({
+    queryKey: ['display-example', debouncedOptions],
+    queryFn: () => fetchExample(debouncedOptions),
+    staleTime: 5 * 60 * 1000, // 5 minutes
+  })
+
+  return (
+    <div
+      data-component="display-options-preview"
+      className={css({
+        p: '4',
+        bg: 'white',
+        rounded: 'xl',
+        border: '2px solid',
+        borderColor: 'brand.200',
+        display: 'flex',
+        flexDirection: 'column',
+        gap: '2',
+      })}
+    >
+      <div
+        className={css({
+          fontSize: 'xs',
+          fontWeight: 'semibold',
+          color: 'gray.500',
+          textTransform: 'uppercase',
+          letterSpacing: 'wider',
+        })}
+      >
+        Preview
+      </div>
+
+      {isLoading ? (
+        <div
+          className={css({
+            display: 'flex',
+            alignItems: 'center',
+            justifyContent: 'center',
+            minH: '200px',
+            color: 'gray.400',
+            fontSize: 'sm',
+          })}
+        >
+          Generating preview...
+        </div>
+      ) : svg ? (
+        <div
+          className={css({
+            display: 'flex',
+            alignItems: 'center',
+            justifyContent: 'center',
+            minH: '200px',
+            '& svg': {
+              maxW: 'full',
+              h: 'auto',
+            },
+          })}
+          dangerouslySetInnerHTML={{ __html: svg }}
+        />
+      ) : null}
+    </div>
+  )
+}