Add SmartSpellChecker + refactor vocab-worksheet page.tsx

SmartSpellChecker (klausur-service):
- Language-aware OCR post-correction without LLMs
- Dual-dictionary heuristic for EN/DE language detection
- Context-based a/I disambiguation via bigram lookup
- Multi-digit substitution (sch00l→school)
- Cross-language guard (don't false-correct DE words in EN column)
- Umlaut correction (Schuler→Schüler, uber→über)
- Integrated into spell_review_entries_sync() pipeline
- 31 tests, 9ms/100 corrections

Vocab-worksheet refactoring (studio-v2):
- Split 2337-line page.tsx into 14 files
- Custom hook useVocabWorksheet.ts (all state + logic)
- 9 components in components/ directory
- types.ts, constants.ts for shared definitions

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

studio-v2/app/vocab-worksheet/components/QRCodeModal.tsx

@@ -0,0 +1,31 @@
+'use client'
+
+import React from 'react'
+import { QRCodeUpload } from '@/components/QRCodeUpload'
+import type { VocabWorksheetHook } from '../types'
+
+export function QRCodeModal({ h }: { h: VocabWorksheetHook }) {
+  const { isDark } = h
+
+  return (
+    <div className="fixed inset-0 z-50 flex items-center justify-center p-4">
+      <div className="absolute inset-0 bg-black/50 backdrop-blur-sm" onClick={() => h.setShowQRModal(false)} />
+      <div className={`relative w-full max-w-md rounded-3xl ${
+        isDark ? 'bg-slate-900' : 'bg-white'
+      }`}>
+        <QRCodeUpload
+          sessionId={h.uploadSessionId}
+          onClose={() => h.setShowQRModal(false)}
+          onFilesChanged={(files) => {
+            h.setMobileUploadedFiles(files)
+            if (files.length > 0) {
+              h.setSelectedMobileFile(files[files.length - 1])
+              h.setDirectFile(null)
+              h.setSelectedDocumentId(null)
+            }
+          }}
+        />
+      </div>
+    </div>
+  )
+}