fix: 手机保存图片加长按保存 fallback

navigator.share 在部分手机浏览器不可用时，改为弹出全屏图片
让用户长按保存到相册。三层 fallback:
1. navigator.share({files}) → 系统分享面板
2. 图片弹窗 → 长按保存（手机通用）
3. 下载链接（桌面）

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

frontend/src/composables/useSaveImage.js

@@ -1,33 +1,40 @@
 /**
- * Save image — on mobile use native share (save to photos),
- * on desktop trigger download.
+ * Save image utility — handles mobile (share/long-press) and desktop (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
+ * Save from a data URL.
+ * Mobile: try navigator.share, fallback to showing image for long-press save.
+ * Desktop: download link.
  */
 export async function saveImageFromUrl(dataUrl, filename) {
-  if (isMobile() && navigator.canShare) {
+  // Mobile: try native share first
+  if (isMobile() && navigator.share && 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
+        return 'shared'
       }
     } catch {}
   }
-  // Desktop fallback
+
+  // Mobile fallback: show image popup for long-press save
+  if (isMobile()) {
+    showImagePopup(dataUrl, filename)
+    return 'popup'
+  }
+
+  // Desktop: direct download
   const a = document.createElement('a')
   a.href = dataUrl
   a.download = filename + '.png'
   a.click()
-  return true
+  return 'downloaded'
 }
 
 /**
@@ -37,40 +44,36 @@ 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')
+  buttons.forEach(b => { b._prevDisplay = b.style.display; 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 = '')
+    buttons.forEach(b => b.style.display = b._prevDisplay || '')
+    const dataUrl = canvas.toDataURL('image/png')
+    return saveImageFromUrl(dataUrl, filename)
+  } catch (e) {
+    buttons.forEach(b => b.style.display = b._prevDisplay || '')
+    console.error('captureAndSave failed:', e)
     return false
   }
 }
 
 /**
- * Save a canvas element as image.
+ * Show a fullscreen image popup for long-press saving (mobile fallback).
  */
-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
+function showImagePopup(dataUrl, filename) {
+  const overlay = document.createElement('div')
+  overlay.style.cssText = 'position:fixed;inset:0;z-index:9999;background:rgba(0,0,0,0.8);display:flex;flex-direction:column;align-items:center;padding:16px;overflow-y:auto'
+  overlay.onclick = () => overlay.remove()
+  overlay.innerHTML = `
+    <div style="color:white;font-size:13px;text-align:center;margin:12px 0;flex-shrink:0" onclick="event.stopPropagation()">
+      📱 长按下方图片 → 保存到相册
+    </div>
+    <img src="${dataUrl}" style="max-width:100%;border-radius:12px;box-shadow:0 4px 20px rgba(0,0,0,0.3)" onclick="event.stopPropagation()">
+    <button onclick="this.parentElement.remove()" style="margin-top:12px;padding:8px 24px;border-radius:20px;border:1px solid rgba(255,255,255,0.3);background:transparent;color:white;cursor:pointer;font-size:14px;flex-shrink:0">关闭</button>
+  `
+  document.body.appendChild(overlay)
 }