fix: 不在打开卡片时自动跑html2canvas，改为保存时按需生成

根本原因: html2canvas在CI headless Electron环境会无限挂起，
导致recipe-detail测试卡死。改为只在用户点"保存图片"时才生成。

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

frontend/src/components/RecipeDetailOverlay.vue

@@ -829,7 +829,7 @@ onMounted(() => {
   else { selectedVolume.value = 'custom'; customVolumeValue.value = Math.round(ml) }
 
   loadBrand()
-  nextTick(() => generateCardImage())
+  // Don't auto-generate card image on mount — generate on demand when saving
 })
 
 function addIngredient() {