fix: 手机保存图片使用 navigator.share 直接保存到相册

- 新增 composables/useSaveImage.js
  - saveImageFromUrl: data URL → 手机分享/桌面下载
  - captureAndSave: DOM元素 → html2canvas → 保存
  - saveCanvasImage: canvas → 保存
- RecipeDetailOverlay: saveImage 改用 saveImageFromUrl
- OilReference: saveModalImage 改用 captureAndSave
- 手机端调用 navigator.share({files}) 弹出系统分享面板

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

frontend/src/components/RecipeDetailOverlay.vue

@@ -583,11 +583,14 @@ async function saveImage() {
     await generateCardImage()
   }
   if (!cardImageUrl.value) return
-  const link = document.createElement('a')
-  link.download = `${recipe.value.name || '配方'}_配方卡.png`
-  link.href = cardImageUrl.value
-  link.click()
-  ui.showToast('已保存图片')
+  const filename = `${recipe.value.name || '配方'}_配方卡`
+  try {
+    const { saveImageFromUrl } = await import('../composables/useSaveImage')
+    await saveImageFromUrl(cardImageUrl.value, filename)
+    ui.showToast('已保存图片')
+  } catch {
+    ui.showToast('保存失败')
+  }
 }
 
 function copyText() {