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/composables/useSaveImage.js

@@ -0,0 +1,76 @@
+/**
+ * Save image — on mobile use native share (save to photos),
+ * on desktop trigger download.
+ */
+
+const isMobile = () => /iPhone|iPad|iPod|Android/i.test(navigator.userAgent)
+
+/**
+ * Save a data URL or blob as image.
+ * Mobile: navigator.share → save to photos
+ * Desktop: download link
+ */
+export async function saveImageFromUrl(dataUrl, filename) {
+  if (isMobile() && navigator.canShare) {
+    try {
+      const res = await fetch(dataUrl)
+      const blob = await res.blob()
+      const file = new File([blob], filename + '.png', { type: 'image/png' })
+      if (navigator.canShare({ files: [file] })) {
+        await navigator.share({ files: [file] })
+        return true
+      }
+    } catch {}
+  }
+  // Desktop fallback
+  const a = document.createElement('a')
+  a.href = dataUrl
+  a.download = filename + '.png'
+  a.click()
+  return true
+}
+
+/**
+ * Capture a DOM element as image and save it.
+ */
+export async function captureAndSave(element, filename) {
+  const { default: html2canvas } = await import('html2canvas')
+  // Hide buttons during capture
+  const buttons = element.querySelectorAll('button')
+  buttons.forEach(b => b.style.display = 'none')
+  try {
+    const canvas = await html2canvas(element, {
+      scale: 2,
+      backgroundColor: '#ffffff',
+      useCORS: true,
+    })
+    buttons.forEach(b => b.style.display = '')
+    return saveCanvasImage(canvas, filename)
+  } catch {
+    buttons.forEach(b => b.style.display = '')
+    return false
+  }
+}
+
+/**
+ * Save a canvas element as image.
+ */
+export async function saveCanvasImage(canvas, filename) {
+  if (isMobile() && navigator.canShare) {
+    try {
+      const blob = await new Promise(r => canvas.toBlob(r, 'image/png'))
+      const file = new File([blob], filename + '.png', { type: 'image/png' })
+      if (navigator.canShare({ files: [file] })) {
+        await navigator.share({ files: [file] })
+        return true
+      }
+    } catch {}
+  }
+  // Desktop fallback
+  const url = canvas.toDataURL('image/png')
+  const a = document.createElement('a')
+  a.href = url
+  a.download = filename + '.png'
+  a.click()
+  return true
+}