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/page.tsx

@@ -8,6 +8,7 @@ import { PageSelection } from './components/PageSelection'
 import { VocabularyTab } from './components/VocabularyTab'
 import { WorksheetTab } from './components/WorksheetTab'
 import { ExportTab } from './components/ExportTab'
+import { SpreadsheetTab } from './components/SpreadsheetTab'
 import { OcrSettingsPanel } from './components/OcrSettingsPanel'
 import { FullscreenPreview } from './components/FullscreenPreview'
 import { QRCodeModal } from './components/QRCodeModal'
@@ -144,6 +145,7 @@ export default function VocabWorksheetPage() {
           {!session && <UploadScreen h={h} />}
           {session && activeTab === 'pages' && <PageSelection h={h} />}
           {session && activeTab === 'vocabulary' && <VocabularyTab h={h} />}
+          {session && activeTab === 'spreadsheet' && <SpreadsheetTab h={h} />}
           {session && activeTab === 'worksheet' && <WorksheetTab h={h} />}
           {session && activeTab === 'export' && <ExportTab h={h} />}
 
@@ -151,7 +153,7 @@ export default function VocabWorksheetPage() {
           {session && activeTab !== 'pages' && (
             <div className={`mt-6 border-t pt-4 ${isDark ? 'border-white/10' : 'border-black/5'}`}>
               <div className="flex justify-center gap-2">
-                {(['vocabulary', 'worksheet', 'export'] as TabId[]).map((tab) => (
+                {(['vocabulary', 'spreadsheet', 'worksheet', 'export'] as TabId[]).map((tab) => (
                   <button
                     key={tab}
                     onClick={() => h.setActiveTab(tab)}
@@ -161,7 +163,7 @@ export default function VocabWorksheetPage() {
                         : (isDark ? 'bg-white/10 text-white/80 hover:bg-white/20' : 'bg-slate-100 text-slate-700 hover:bg-slate-200')
                     }`}
                   >
-                    {tab === 'vocabulary' ? 'Vokabeln' : tab === 'worksheet' ? 'Arbeitsblatt' : 'Export'}
+                    {tab === 'vocabulary' ? 'Vokabeln' : tab === 'spreadsheet' ? 'Spreadsheet' : tab === 'worksheet' ? 'Arbeitsblatt' : 'Export'}
                   </button>
                 ))}
               </div>