feat(vocabService): enhance answer validation with comparable variants

- Introduced methods to expand comparable answer variants and improve answer equivalence checks, allowing for more flexible user responses.
- Updated the answer validation logic to utilize the new methods, enhancing the accuracy of response evaluation during exercises.

backend/services/vocabService.js

@@ -500,6 +500,33 @@ export default class VocabService {
     return normalized.replace(/\s+/g, '');
   }
 
+  _expandComparableAnswerVariants(text) {
+    const raw = String(text || '').trim();
+    if (!raw) return [];
+    const variants = new Set([raw]);
+
+    // Beispiel: "heute (heute am Tag)" -> "heute" und "heute am Tag"
+    const withoutParentheses = raw.replace(/\s*\([^)]*\)\s*/g, ' ').replace(/\s+/g, ' ').trim();
+    if (withoutParentheses) variants.add(withoutParentheses);
+
+    const parenMatches = raw.match(/\(([^)]+)\)/g) || [];
+    parenMatches.forEach((chunk) => {
+      const content = chunk.replace(/[()]/g, '').trim();
+      if (content) variants.add(content);
+    });
+
+    return Array.from(variants);
+  }
+
+  _isEquivalentAnswer(userAnswer, canonicalAnswer) {
+    const normalizedUser = this._normalizeTextAnswer(userAnswer);
+    if (!normalizedUser) return false;
+    const canonicalVariants = this._expandComparableAnswerVariants(canonicalAnswer)
+      .map((entry) => this._normalizeTextAnswer(entry))
+      .filter(Boolean);
+    return canonicalVariants.includes(normalizedUser);
+  }
+
   _parseExercisePayload(value) {
     if (!value) return {};
     if (typeof value === 'string') {
@@ -3550,7 +3577,7 @@ export default class VocabService {
       if (typeof userAnswer === 'string') {
         const u = norm(userAnswer);
         if (!u) return false;
-        return correctTexts.some((t) => norm(t) === u);
+        return correctTexts.some((t) => this._isEquivalentAnswer(userAnswer, t));
       }
 
       // Legacy: Index in die gespeicherten (nicht gemischten) Optionen