feat(VocabService, VocabPracticeDialog, VocabLessonView): enhance vocabulary training logic and UI feedback

- Introduced methods for improved text analysis and validation in VocabService, including `_wordCount` and `_looksLikeFragmentMismatch`, to better assess learning and reference pairs.
- Updated VocabPracticeDialog to display submitted answers and correct solutions, enhancing user feedback during practice sessions.
- Enhanced VocabLessonView to ensure only trainable vocabulary pairs are processed, improving the quality of vocabulary training.
- Added localization entries for new UI elements in both English and German, ensuring clarity in user interactions.

backend/services/vocabService.js

@@ -92,9 +92,35 @@ export default class VocabService {
       return false;
     }
 
+    if (this._looksLikeFragmentMismatch(learning, reference)) {
+      return false;
+    }
+
     return !this._isInstructionLikeText(learning) && !this._isInstructionLikeText(reference);
   }
 
+  _wordCount(value) {
+    return String(value || '')
+      .trim()
+      .replace(/[\p{P}\p{S}]+/gu, ' ')
+      .split(/\s+/)
+      .filter(Boolean)
+      .length;
+  }
+
+  _looksLikeFragmentMismatch(left, right) {
+    const leftWords = this._wordCount(left);
+    const rightWords = this._wordCount(right);
+    const leftText = String(left || '').trim();
+    const rightText = String(right || '').trim();
+    const leftLooksSentence = leftWords >= 3 || /[?!.].+\S/.test(leftText);
+    const rightLooksSentence = rightWords >= 3 || /[?!.].+\S/.test(rightText);
+    const leftLooksShortFragment = leftWords <= 1 && leftText.length <= 12;
+    const rightLooksShortFragment = rightWords <= 1 && rightText.length <= 12;
+
+    return (leftLooksShortFragment && rightLooksSentence) || (rightLooksShortFragment && leftLooksSentence);
+  }
+
   _calculateSrsSchedule(item, { correct, rating = null } = {}) {
     const now = new Date();
     const previousStage = Math.max(0, Number(item?.stage) || 0);