fix: 知识卡保存图片参数与配方卡完全一致，去掉长按fallback

- html2canvas 参数: backgroundColor=null, scale=3, allowTaint=false
  （与 RecipeDetailOverlay.generateCardImage 完全一致）
- useSaveImage 精简为只有 share + download 两条路径
- 截图失败时显示具体错误信息便于调试

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

frontend/src/views/OilReference.vue

@@ -727,24 +727,33 @@ function exportPDF() {
   setTimeout(() => w.print(), 500)
 }
 
-// Save element as image — same flow as recipe card:
-// 1. html2canvas → dataUrl  2. saveImageFromUrl (share/popup/download)
+// Save element as image — identical to recipe card flow
 async function saveElementAsImage(el, name) {
   if (!el) { ui.showToast('找不到卡片'); return }
+  const html2canvas = (await import('html2canvas')).default
+  const { saveImageFromUrl } = await import('../composables/useSaveImage')
+
+  // Hide buttons during capture
+  const btns = el.querySelectorAll('button')
+  btns.forEach(b => { b._d = b.style.display; b.style.display = 'none' })
+
   try {
-    const html2canvas = (await import('html2canvas')).default
-    const { saveImageFromUrl } = await import('../composables/useSaveImage')
-    // Hide buttons during capture
-    const btns = el.querySelectorAll('button')
-    btns.forEach(b => { b._d = b.style.display; b.style.display = 'none' })
-    const canvas = await html2canvas(el, { scale: 2, backgroundColor: '#fff', useCORS: true })
+    // Same params as recipe card's generateCardImage
+    const canvas = await html2canvas(el, {
+      backgroundColor: null,
+      scale: 3,
+      useCORS: true,
+      allowTaint: false,
+    })
     btns.forEach(b => b.style.display = b._d || '')
+
     const dataUrl = canvas.toDataURL('image/png')
-    const result = await saveImageFromUrl(dataUrl, name)
-    if (result === 'shared' || result === 'downloaded') ui.showToast('图片已保存')
+    await saveImageFromUrl(dataUrl, name)
+    ui.showToast('图片已保存')
   } catch (e) {
-    console.error('saveElementAsImage:', e)
-    ui.showToast('保存失败')
+    btns.forEach(b => b.style.display = b._d || '')
+    console.error('saveElementAsImage failed:', e)
+    ui.showToast('截图失败: ' + (e.message || '未知错误'))
   }
 }