fix: 配方保存时持久化volume字段，套装对比页复用容量显示逻辑

saveRecipe payload 漏传 selectedVolume 导致编辑器选择的容量从未写入数据库；
套装方案对比页改用与 RecipeCard 一致的 volumeLabel 计算逻辑。

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/RecipeDetailOverlay.vue

@@ -1010,6 +1010,7 @@ async function saveRecipe() {
       note: editNote.value.trim(),
       tags: editTags.value,
       ingredients: allIngs,
+      volume: selectedVolume.value || '',
     }
     await recipesStore.saveRecipe(payload)
     // Reload recipes so the data is fresh when re-opened