feat(vocab): improve vocabulary preparation logic in VocabLessonView

- Refactored prepItems method to ensure only valid target-gloss pairs are included, enhancing the quality of vocabulary preparation.
- Introduced a mechanism to limit the output to a maximum of 30 unique pairs, improving the compactness of the preparation round.
- Enhanced handling of core patterns and extracted vocabulary to prioritize high-quality learning materials.

frontend/src/views/social/VocabLessonView.vue

@@ -1296,12 +1296,32 @@ export default {
         .filter(Boolean);
     },
     prepItems() {
-      return this.lessonVocab
-        .map((item) => ({
-          target: String(item?.reference || '').trim(),
-          gloss: String(item?.learning || '').trim()
-        }))
-        .filter((item) => item.target);
+      // Vorbereitung nur mit echten Paaren (Zielsprache + Übersetzung),
+      // damit weder leere Gloss-Zeilen noch übergroße Listen entstehen.
+      const out = [];
+      const seen = new Set();
+      const pushUnique = (target, gloss) => {
+        const t = String(target || '').trim();
+        const g = String(gloss || '').trim();
+        if (!t || !g) return;
+        const key = `${this.normalizeLessonVocabTerm(t)}|${this.normalizeLessonVocabTerm(g)}`;
+        if (seen.has(key)) return;
+        seen.add(key);
+        out.push({ target: t, gloss: g });
+      };
+
+      // 1) Didaktisch gepflegte Kernmuster zuerst (höchste Qualität)
+      this.normalizedCorePatterns.forEach((p) => {
+        pushUnique(p?.target, p?.gloss);
+      });
+
+      // 2) Ergänzung aus extrahierten Übungsvokabeln, aber nur saubere Paare
+      this.lessonVocab.forEach((item) => {
+        pushUnique(item?.reference, item?.learning);
+      });
+
+      // Begrenzen, damit die Vorbereitungsrunde kompakt bleibt
+      return out.slice(0, 30);
     },
     currentPrepItem() {
       return this.prepItems[this.lessonPrepIndex] || null;