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/components/dashboard/LearningProgress.tsx

@@ -31,11 +31,11 @@ interface LearningProgressProps {
   glassCard: 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 function LearningProgress({ isDark, glassCard }: LearningProgressProps) {
@@ -51,8 +51,8 @@ export function LearningProgress({ isDark, glassCard }: LearningProgressProps) {
     setIsLoading(true)
     try {
       const [unitsResp, progressResp] = await Promise.all([
-        fetch(`${getBackendUrl()}/api/learning-units/`),
-        fetch(`${getBackendUrl()}/api/progress/`),
+        fetch(`${getApiBase()}/api/learning-units/`),
+        fetch(`${getApiBase()}/api/progress/`),
       ])
 
       if (unitsResp.ok) setUnits(await unitsResp.json())