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/vocabulary/page.tsx

@@ -22,11 +22,9 @@ interface VocabWord {
   tags: 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`
+/** Use Next.js API proxy to avoid mixed-content/CORS issues */
+function getApiBase() {
+  return ''  // Same-origin: /api/vocabulary/... proxied by Next.js
 }
 
 export default function VocabularyPage() {
@@ -55,7 +53,7 @@ export default function VocabularyPage() {
 
   // Load filters on mount
   useEffect(() => {
-    fetch(`${getBackendUrl()}/api/vocabulary/filters`)
+    fetch(`${getApiBase()}/api/vocabulary/filters`)
       .then(r => r.ok ? r.json() : null)
       .then(d => { if (d) setFilters(d) })
       .catch(() => {})
@@ -73,12 +71,12 @@ export default function VocabularyPage() {
       try {
         let url: string
         if (query.trim()) {
-          url = `${getBackendUrl()}/api/vocabulary/search?q=${encodeURIComponent(query)}&limit=30`
+          url = `${getApiBase()}/api/vocabulary/search?q=${encodeURIComponent(query)}&limit=30`
         } else {
           const params = new URLSearchParams({ limit: '30' })
           if (posFilter) params.set('pos', posFilter)
           if (diffFilter) params.set('difficulty', String(diffFilter))
-          url = `${getBackendUrl()}/api/vocabulary/browse?${params}`
+          url = `${getApiBase()}/api/vocabulary/browse?${params}`
         }
         const resp = await fetch(url)
         if (resp.ok) {
@@ -107,7 +105,7 @@ export default function VocabularyPage() {
     if (!unitTitle.trim() || selectedWords.length === 0) return
     setIsCreating(true)
     try {
-      const resp = await fetch(`${getBackendUrl()}/api/vocabulary/units`, {
+      const resp = await fetch(`${getApiBase()}/api/vocabulary/units`, {
         method: 'POST',
         headers: { 'Content-Type': 'application/json' },
         body: JSON.stringify({