Fix: Use Next.js API proxy to avoid mixed-content/CORS errors

HTTPS pages cannot fetch from HTTP backend ports. Added Next.js
API route proxies for /api/vocabulary, /api/learning-units, /api/progress
that forward to backend-lehrer internally (same Docker network, HTTP).

All frontend pages now use same-origin requests (getApiBase = '')
instead of direct port:8001 connections.

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

studio-v2/app/learn/[unitId]/flashcards/page.tsx

@@ -15,11 +15,11 @@ interface QAItem {
   incorrect_count: number
 }
 
-function getBackendUrl() {
-  if (typeof window === 'undefined') return 'http://localhost:8001'
-  const { hostname, protocol } = window.location
-  if (hostname === 'localhost') return 'http://localhost:8001'
-  return `${protocol}//${hostname}:8001`
+function getApiBase() {
+  return ''  // Same-origin proxy
+  
+  
+  
 }
 
 export default function FlashcardsPage() {
@@ -45,7 +45,7 @@ export default function FlashcardsPage() {
   const loadQA = async () => {
     setIsLoading(true)
     try {
-      const resp = await fetch(`${getBackendUrl()}/api/learning-units/${unitId}/qa`)
+      const resp = await fetch(`${getApiBase()}/api/learning-units/${unitId}/qa`)
       if (!resp.ok) throw new Error(`HTTP ${resp.status}`)
       const data = await resp.json()
       setItems(data.qa_items || [])
@@ -63,7 +63,7 @@ export default function FlashcardsPage() {
     // Update Leitner progress
     try {
       await fetch(
-        `${getBackendUrl()}/api/learning-units/${unitId}/leitner/update?item_id=${item.id}&correct=${correct}`,
+        `${getApiBase()}/api/learning-units/${unitId}/leitner/update?item_id=${item.id}&correct=${correct}`,
         { method: 'POST' }
       )
     } catch (err) {

studio-v2/app/learn/[unitId]/quiz/page.tsx

@@ -13,11 +13,11 @@ interface MCQuestion {
   explanation?: string
 }
 
-function getBackendUrl() {
-  if (typeof window === 'undefined') return 'http://localhost:8001'
-  const { hostname, protocol } = window.location
-  if (hostname === 'localhost') return 'http://localhost:8001'
-  return `${protocol}//${hostname}:8001`
+function getApiBase() {
+  return ''  // Same-origin proxy
+  
+  
+  
 }
 
 export default function QuizPage() {
@@ -43,7 +43,7 @@ export default function QuizPage() {
   const loadMC = async () => {
     setIsLoading(true)
     try {
-      const resp = await fetch(`${getBackendUrl()}/api/learning-units/${unitId}/mc`)
+      const resp = await fetch(`${getApiBase()}/api/learning-units/${unitId}/mc`)
       if (!resp.ok) throw new Error(`HTTP ${resp.status}`)
       const data = await resp.json()
       setQuestions(data.questions || [])

studio-v2/app/learn/[unitId]/story/page.tsx

@@ -5,16 +5,16 @@ import { useParams, useRouter } from 'next/navigation'
 import { useTheme } from '@/lib/ThemeContext'
 import { AudioButton } from '@/components/learn/AudioButton'
 
-function getBackendUrl() {
-  if (typeof window === 'undefined') return 'http://localhost:8001'
-  const { hostname, protocol } = window.location
-  if (hostname === 'localhost') return 'http://localhost:8001'
-  return `${protocol}//${hostname}:8001`
+function getApiBase() {
+  return ''  // Same-origin proxy
+  
+  
+  
 }
 
 function getKlausurApiUrl() {
   if (typeof window === 'undefined') return 'http://localhost:8086'
-  const { hostname, protocol } = window.location
+  
   if (hostname === 'localhost') return 'http://localhost:8086'
   return `${protocol}//${hostname}/klausur-api`
 }
@@ -39,7 +39,7 @@ export default function StoryPage() {
 
     try {
       // First get the QA data to extract vocabulary
-      const qaResp = await fetch(`${getBackendUrl()}/api/learning-units/${unitId}/qa`)
+      const qaResp = await fetch(`${getApiBase()}/api/learning-units/${unitId}/qa`)
       let vocabulary: { english: string; german: string }[] = []
 
       if (qaResp.ok) {
@@ -58,7 +58,7 @@ export default function StoryPage() {
       }
 
       // Generate story
-      const resp = await fetch(`${getBackendUrl()}/api/learning-units/${unitId}/generate-story`, {
+      const resp = await fetch(`${getApiBase()}/api/learning-units/${unitId}/generate-story`, {
         method: 'POST',
         headers: { 'Content-Type': 'application/json' },
         body: JSON.stringify({ vocabulary, language, grade_level: '5-8' }),

studio-v2/app/learn/[unitId]/type/page.tsx

@@ -13,11 +13,11 @@ interface QAItem {
   leitner_box: number
 }
 
-function getBackendUrl() {
-  if (typeof window === 'undefined') return 'http://localhost:8001'
-  const { hostname, protocol } = window.location
-  if (hostname === 'localhost') return 'http://localhost:8001'
-  return `${protocol}//${hostname}:8001`
+function getApiBase() {
+  return ''  // Same-origin proxy
+  
+  
+  
 }
 
 export default function TypePage() {
@@ -44,7 +44,7 @@ export default function TypePage() {
   const loadQA = async () => {
     setIsLoading(true)
     try {
-      const resp = await fetch(`${getBackendUrl()}/api/learning-units/${unitId}/qa`)
+      const resp = await fetch(`${getApiBase()}/api/learning-units/${unitId}/qa`)
       if (!resp.ok) throw new Error(`HTTP ${resp.status}`)
       const data = await resp.json()
       setItems(data.qa_items || [])
@@ -61,7 +61,7 @@ export default function TypePage() {
 
     try {
       await fetch(
-        `${getBackendUrl()}/api/learning-units/${unitId}/leitner/update?item_id=${item.id}&correct=${correct}`,
+        `${getApiBase()}/api/learning-units/${unitId}/leitner/update?item_id=${item.id}&correct=${correct}`,
         { method: 'POST' }
       )
     } catch (err) {