feat(VocabPracticeDialog): enhance answer variant expansion logic

- Renamed `expandAnswerVariants` to `expandSingleAnswerVariants` for clarity and added a new `expandAnswerVariants` method to handle full-answer alternatives like "A / B".
- Improved the logic to ensure that phrase alternatives are expanded correctly, enhancing the accuracy of answer variant generation during vocabulary practice.

frontend/src/dialogues/socialnetwork/VocabPracticeDialog.vue

@@ -719,7 +719,7 @@ export default {
       }
       return Array.from(answers);
     },
-    expandAnswerVariants(answer) {
+    expandSingleAnswerVariants(answer) {
       const base = String(answer || '').trim();
       if (!base) return [];
 
@@ -760,6 +760,24 @@ export default {
 
       return [...new Set([base, ...variants])];
     },
+    expandAnswerVariants(answer) {
+      const base = String(answer || '').trim();
+      if (!base) return [];
+
+      // Handle full-answer alternatives like "A / B" as separate valid answers.
+      // Word-level slash expansion alone does not cover this reliably.
+      const phraseAlternatives = base
+        .split(/\s+\/\s+/)
+        .map((part) => String(part || '').trim())
+        .filter(Boolean);
+
+      if (phraseAlternatives.length >= 2) {
+        const expanded = phraseAlternatives.flatMap((part) => this.expandSingleAnswerVariants(part));
+        return [...new Set(expanded)];
+      }
+
+      return this.expandSingleAnswerVariants(base);
+    },
     computeWeight(item) {
       const st = this.perId[item.id] || { c: 0, w: 0, streak: 0, lastAsked: 0 };
       let w = 1;