feat: add single-page worksheet preview API endpoint

Add new API route for fetching individual worksheet pages on-demand:
- Route: POST /api/create/worksheets/preview/[pageNumber]
- Uses existing generateSinglePage() function
- Returns { page, totalPages } for the requested page
- Validates page number parameter

This endpoint enables on-demand page loading for large worksheets.

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

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

apps/web/src/app/api/create/worksheets/preview/[pageNumber]/route.ts

@@ -0,0 +1,52 @@
+// API route for generating a single worksheet page (SVG)
+
+import { type NextRequest, NextResponse } from 'next/server'
+import { generateSinglePage } from '@/app/create/worksheets/generatePreview'
+import type { WorksheetFormState } from '@/app/create/worksheets/types'
+
+export const dynamic = 'force-dynamic'
+
+export async function POST(
+  request: NextRequest,
+  { params }: { params: { pageNumber: string } }
+) {
+  try {
+    const body: WorksheetFormState = await request.json()
+    const pageNumber = parseInt(params.pageNumber, 10)
+
+    if (isNaN(pageNumber) || pageNumber < 0) {
+      return NextResponse.json({ error: 'Invalid page number' }, { status: 400 })
+    }
+
+    // Generate only the requested page
+    const result = generateSinglePage(body, pageNumber)
+
+    if (!result.success) {
+      return NextResponse.json(
+        {
+          error: result.error,
+          details: result.details,
+        },
+        { status: result.error?.includes('Invalid page number') ? 404 : 400 }
+      )
+    }
+
+    // Return the page and total count
+    return NextResponse.json({
+      page: result.page,
+      totalPages: result.totalPages,
+    })
+  } catch (error) {
+    console.error('Error generating page preview:', error)
+
+    const errorMessage = error instanceof Error ? error.message : String(error)
+
+    return NextResponse.json(
+      {
+        error: 'Failed to generate page preview',
+        message: errorMessage,
+      },
+      { status: 500 }
+    )
+  }
+}