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.

frontend/src/views/social/VocabLessonView.vue

@@ -1452,7 +1452,7 @@ export default {
         const oriented = orientPair(entry?.learning, entry?.reference);
         const reference = String(oriented.reference || '').trim();
         const learning = String(oriented.learning || '').trim();
-        if (!reference) return;
+        if (!this.isTrainableLessonVocabPair(learning, reference)) return;
         const key = this.normalizeLessonVocabTerm(reference);
         if (!vocabByReference.has(key)) {
           const variants = new Set();
@@ -1523,7 +1523,7 @@ export default {
       return { from, to, page, pages: totalPages, total: n };
     },
     trainableLessonVocab() {
-      return this.lessonVocab.filter((entry) => entry.learning && entry.reference && entry.learning !== entry.reference);
+      return this.lessonVocab.filter((entry) => this.isTrainableLessonVocabPair(entry.learning, entry.reference));
     },
     lessonDidactics() {
       return this.lesson?.didactics || {
@@ -1610,7 +1610,7 @@ export default {
         const oriented = orientPair(target, gloss);
         const t = String(oriented.target || '').trim();
         const g = String(oriented.gloss || '').trim();
-        if (!t || !g) return;
+        if (!this.isTrainableLessonVocabPair(g, t)) return;
         const key = `${this.normalizeLessonVocabTerm(t)}|${this.normalizeLessonVocabTerm(g)}`;
         if (seen.has(key)) return;
         seen.add(key);
@@ -2125,6 +2125,34 @@ export default {
         .replace(/^[.,!?;:]+|[.,!?;:]+$/g, '')
         .trim();
     },
+    lessonVocabWordCount(value) {
+      return String(value || '')
+        .trim()
+        .replace(/[\p{P}\p{S}]+/gu, ' ')
+        .split(/\s+/)
+        .filter(Boolean)
+        .length;
+    },
+    looksLikeLessonVocabFragmentMismatch(left, right) {
+      const leftWords = this.lessonVocabWordCount(left);
+      const rightWords = this.lessonVocabWordCount(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);
+    },
+    isTrainableLessonVocabPair(learning, reference) {
+      const l = String(learning || '').trim();
+      const r = String(reference || '').trim();
+      if (!l || !r || this.normalizeLessonVocabTerm(l) === this.normalizeLessonVocabTerm(r)) {
+        return false;
+      }
+      return !this.looksLikeLessonVocabFragmentMismatch(l, r);
+    },
     normalizeCorePatternEntry(p) {
       if (p && typeof p === 'object' && p.target) {
         return {