feat: refine exercise focus area retrieval in training planning

- Updated the SQL query in the training planning module to fetch the primary exercise focus area more efficiently.
- Enhanced error handling in the API utility to provide clearer feedback on potential issues, including CORS and URL errors, improving debugging for developers.

backend/routers/training_planning.py

@@ -117,7 +117,13 @@ def _fetch_sections(cur, unit_id: int) -> List[Dict[str, Any]]:
             SELECT tusi.*,
                 e.title AS exercise_title,
                 e.summary AS exercise_summary,
-                e.focus_area AS exercise_focus_area,
+                (
+                    SELECT fa.name FROM exercise_focus_areas efa
+                    JOIN focus_areas fa ON fa.id = efa.focus_area_id
+                    WHERE efa.exercise_id = e.id
+                    ORDER BY efa.is_primary DESC NULLS LAST, fa.name ASC
+                    LIMIT 1
+                ) AS exercise_focus_area,
                 ev.variant_name AS exercise_variant_name
             FROM training_unit_section_items tusi
             LEFT JOIN exercises e ON tusi.exercise_id = e.id

frontend/src/utils/api.js

@@ -43,7 +43,9 @@ async function request(endpoint, options = {}) {
         API_URL && API_URL.length > 0
           ? `Verbindung zum API unter ${API_URL} fehlgeschlagen. Läuft das Backend (z. B. Port 8098) und ist CORS erlaubt?`
           : 'Kein VITE_API_URL gesetzt: Anfragen gehen an die Frontend-URL und schlagen oft fehl. Setze in .env z. B. VITE_API_URL=http://localhost:8098 und starte Vite neu.'
-      throw new Error(hint)
+      // Ursache oft: CORS (v. a. bei Fehler-Antworten ohne CORS-Header), Adblocker, falsche URL —
+      // Login kann trotzdem klappen wenn nur eine andere Route betroffen ist.
+      throw new Error(`${hint} [Technisch: ${e.message}; URL war ${endpoint}]`)
     }
     throw e
   }