fix: remove /languparent path prefix and align auth/feed flow with API

- Remove /languparent prefix from all API calls (routes are at /auth/* now)
- Align register/login token handling (JWT returned directly, no registration token)
- Feed rewritten to use new getFeedPairs endpoint with PairSentenceCard/YesNoCard/WordCard
- App.jsx: check language_native_id / language_target_id for profile completeness
- Profil.jsx: add logout button

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/App.jsx

@@ -22,7 +22,7 @@ function AppContent() {
     )
   }
 
-  if (!user || !user.username || !user.language_native || !user.language_target) {
+  if (!user || !user.username || !user.language_native_id || !user.language_target_id) {
     return <AuthScreen />
   }