fix: calculate total problems correctly in preview API

Bug fix:
- Preview API was not calculating `total` field (pages × problemsPerPage)
- This caused only 1 page to generate even when config specified 4 pages
- Now correctly calculates total, rows, and other derived fields

Added debug logging:
- Log problem count and config in generatePreview
- Log Typst sources page count
- Log preview page count in shared worksheet viewer
- These logs help diagnose multi-page rendering issues

This fixes the issue where shared worksheets only showed the first page
even when the config specified multiple pages.

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

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

apps/web/src/app/api/worksheets/preview/route.ts

@@ -17,7 +17,14 @@ export async function POST(request: Request) {
       return NextResponse.json({ success: false, error: 'Missing config' }, { status: 400 })
     }
 
-    // Add date and seed if missing
+    // Calculate derived state fields
+    const problemsPerPage = config.problemsPerPage ?? 20
+    const pages = config.pages ?? 1
+    const cols = config.cols ?? 5
+    const rows = Math.ceil((problemsPerPage * pages) / cols)
+    const total = problemsPerPage * pages
+
+    // Add date, seed, and derived fields if missing
     const fullConfig: WorksheetFormState = {
       ...config,
       date:
@@ -28,6 +35,8 @@ export async function POST(request: Request) {
           year: 'numeric',
         }),
       seed: config.seed || Date.now() % 2147483647,
+      rows,
+      total,
     }
 
     // Generate preview

apps/web/src/app/create/worksheets/generatePreview.ts

@@ -115,7 +115,13 @@ export function generateWorksheetPreview(config: WorksheetFormState): PreviewRes
     }
 
     // Generate Typst sources (one per page)
+    console.log('[generatePreview] Generated problems:', problems.length, 'for config:', {
+      pages: validatedConfig.pages,
+      problemsPerPage: validatedConfig.problemsPerPage,
+      total: validatedConfig.total,
+    })
     const typstSources = generateTypstSource(validatedConfig, problems)
+    console.log('[generatePreview] Generated Typst sources:', typstSources.length, 'pages')
 
     // Compile each page source to SVG (using stdout for single-page output)
     const pages: string[] = []