Add interactive learning modules MVP (Phases 1-3.1)

New feature: After OCR vocabulary extraction, users can generate interactive
learning modules (flashcards, quiz, type trainer) with one click.

Frontend (studio-v2):
- Fortune Sheet spreadsheet editor tab in vocab-worksheet
- "Lernmodule generieren" button in ExportTab
- /learn page with unit overview and exercise type cards
- /learn/[unitId]/flashcards — Flip-card trainer with Leitner spaced repetition
- /learn/[unitId]/quiz — Multiple choice quiz with explanations
- /learn/[unitId]/type — Type-in trainer with Levenshtein distance feedback
- AudioButton component using Web Speech API for EN+DE TTS

Backend (klausur-service):
- vocab_learn_bridge.py: Converts VocabularyEntry[] to analysis_data format
- POST /sessions/{id}/generate-learning-unit endpoint

Backend (backend-lehrer):
- generate-qa, generate-mc, generate-cloze endpoints on learning units
- get-qa/mc/cloze data retrieval endpoints
- Leitner progress update + next review items endpoints

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

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

@@ -0,0 +1,157 @@
+'use client'
+
+/**
+ * SpreadsheetTab — Fortune Sheet editor for vocabulary data.
+ *
+ * Converts VocabularyEntry[] into a Fortune Sheet workbook
+ * where users can edit vocabulary in a familiar Excel-like UI.
+ */
+
+import React, { useMemo, useCallback } from 'react'
+import dynamic from 'next/dynamic'
+import type { VocabWorksheetHook } from '../types'
+
+const Workbook = dynamic(
+  () => import('@fortune-sheet/react').then((m) => m.Workbook),
+  { ssr: false, loading: () => <div className="py-8 text-center text-sm text-gray-400">Spreadsheet wird geladen...</div> },
+)
+
+import '@fortune-sheet/react/dist/index.css'
+
+/** Convert VocabularyEntry[] to Fortune Sheet sheet data */
+function vocabToSheet(vocabulary: VocabWorksheetHook['vocabulary']) {
+  const headers = ['Englisch', 'Deutsch', 'Beispielsatz', 'Wortart', 'Seite']
+  const numCols = headers.length
+  const numRows = vocabulary.length + 1 // +1 for header
+
+  const celldata: any[] = []
+
+  // Header row
+  headers.forEach((label, c) => {
+    celldata.push({
+      r: 0,
+      c,
+      v: { v: label, m: label, bl: 1, bg: '#f0f4ff', fc: '#1e293b' },
+    })
+  })
+
+  // Data rows
+  vocabulary.forEach((entry, idx) => {
+    const r = idx + 1
+    celldata.push({ r, c: 0, v: { v: entry.english, m: entry.english } })
+    celldata.push({ r, c: 1, v: { v: entry.german, m: entry.german } })
+    celldata.push({ r, c: 2, v: { v: entry.example_sentence || '', m: entry.example_sentence || '' } })
+    celldata.push({ r, c: 3, v: { v: entry.word_type || '', m: entry.word_type || '' } })
+    celldata.push({ r, c: 4, v: { v: entry.source_page != null ? String(entry.source_page) : '', m: entry.source_page != null ? String(entry.source_page) : '' } })
+  })
+
+  // Column widths
+  const columnlen: Record<string, number> = {
+    '0': 180, // Englisch
+    '1': 180, // Deutsch
+    '2': 280, // Beispielsatz
+    '3': 100, // Wortart
+    '4': 60,  // Seite
+  }
+
+  // Row heights
+  const rowlen: Record<string, number> = {}
+  rowlen['0'] = 28 // header
+
+  // Borders: light grid
+  const borderInfo = numRows > 0 && numCols > 0 ? [{
+    rangeType: 'range',
+    borderType: 'border-all',
+    color: '#e5e7eb',
+    style: 1,
+    range: [{ row: [0, numRows - 1], column: [0, numCols - 1] }],
+  }] : []
+
+  return {
+    name: 'Vokabeln',
+    id: 'vocab_sheet',
+    celldata,
+    row: numRows,
+    column: numCols,
+    status: 1,
+    config: {
+      columnlen,
+      rowlen,
+      borderInfo,
+    },
+  }
+}
+
+export function SpreadsheetTab({ h }: { h: VocabWorksheetHook }) {
+  const { isDark, glassCard, vocabulary } = h
+
+  const sheets = useMemo(() => {
+    if (!vocabulary || vocabulary.length === 0) return []
+    return [vocabToSheet(vocabulary)]
+  }, [vocabulary])
+
+  const estimatedHeight = Math.max(500, (vocabulary.length + 2) * 26 + 80)
+
+  const handleSaveFromSheet = useCallback(async () => {
+    await h.saveVocabulary()
+  }, [h])
+
+  if (vocabulary.length === 0) {
+    return (
+      <div className={`${glassCard} rounded-2xl p-6`}>
+        <p className={`text-center py-12 ${isDark ? 'text-white/60' : 'text-slate-500'}`}>
+          Keine Vokabeln vorhanden. Bitte zuerst Seiten verarbeiten.
+        </p>
+      </div>
+    )
+  }
+
+  return (
+    <div className={`${glassCard} rounded-2xl p-4`}>
+      <div className="flex items-center justify-between mb-4">
+        <h2 className={`text-lg font-semibold ${isDark ? 'text-white' : 'text-slate-900'}`}>
+          Spreadsheet-Editor
+        </h2>
+        <div className="flex items-center gap-3">
+          <span className={`text-sm ${isDark ? 'text-white/50' : 'text-slate-500'}`}>
+            {vocabulary.length} Vokabeln
+          </span>
+          <button
+            onClick={handleSaveFromSheet}
+            className="px-4 py-2 rounded-xl text-sm font-medium bg-gradient-to-r from-purple-500 to-pink-500 text-white hover:shadow-lg transition-all"
+          >
+            Speichern
+          </button>
+        </div>
+      </div>
+
+      <div
+        className="rounded-xl overflow-hidden border"
+        style={{
+          borderColor: isDark ? 'rgba(255,255,255,0.1)' : 'rgba(0,0,0,0.1)',
+        }}
+      >
+        {sheets.length > 0 && (
+          <div style={{ width: '100%', height: `${estimatedHeight}px` }}>
+            <Workbook
+              data={sheets}
+              lang="en"
+              showToolbar
+              showFormulaBar={false}
+              showSheetTabs={false}
+              toolbarItems={[
+                'undo', 'redo', '|',
+                'font-bold', 'font-italic', 'font-strikethrough', '|',
+                'font-color', 'background', '|',
+                'font-size', '|',
+                'horizontal-align', 'vertical-align', '|',
+                'text-wrap', '|',
+                'border',
+              ]}
+            />
+          </div>
+        )}
+      </div>
+    </div>
+  )
+}