feat: 强化多视角图片一致性 + 修复下载逻辑 + 技术文档

- 新增品类专属背面/侧面描述(BACK_VIEW_HINTS/SIDE_VIEW_HINTS)
- 强化一致性前缀策略，按视角定制相机位置描述
- 更新视角映射提示词为纯摄影术语
- 修复前端下载逻辑：改用fetch直接下载当前视角图片
- HTTPS改HTTP修复外网URL访问
- 新增多视角一致性与3D视频生成技术文档

frontend/src/components/DesignPreview.vue

@@ -195,8 +195,7 @@ import { useRouter } from 'vue-router'
 import { Loading, PictureFilled, ZoomIn, ZoomOut, RefreshRight, Download, User, Platform, VideoCameraFilled } from '@element-plus/icons-vue'
 import { ElMessage } from 'element-plus'
 import type { Design } from '@/stores/design'
-import { getDesignDownloadUrl, generate3DModelApi, generateVideoApi } from '@/api/design'
-import request from '@/api/request'
+import { generate3DModelApi, generateVideoApi } from '@/api/design'
 
 const props = defineProps<{
   design: Design
@@ -384,38 +383,31 @@ const downloadFilename = computed(() => {
   return `${category}${subType ? '-' + subType : ''}${viewSuffix}-${props.design.id}.png`
 })
 
-// 下载设计图
-const handleDownload = () => {
+// 下载设计图（直接下载当前视角的图片，而非固定第一张）
+const handleDownload = async () => {
   const imgUrl = currentImageUrl.value
   if (!imgUrl) {
     ElMessage.error('图片不存在')
     return
   }
-  // 远程 URL 直接新窗口打开（用户右键可保存）
-  if (imgUrl.startsWith('http')) {
-    window.open(imgUrl, '_blank')
-    return
-  }
-  // 本地文件通过 axios 携带 Token 下载
   downloading.value = true
-  request.get(getDesignDownloadUrl(props.design.id), {
-    responseType: 'blob'
-  }).then((response: any) => {
-    const blob = new Blob([response], { type: 'image/png' })
-    const url = window.URL.createObjectURL(blob)
-    const link = document.createElement('a')
-    link.href = url
-    link.download = downloadFilename.value
-    document.body.appendChild(link)
-    link.click()
-    document.body.removeChild(link)
-    window.URL.revokeObjectURL(url)
+  try {
+    // 远程 URL 或本地路径都通过 fetch 下载当前视角的图片
+    const res = await fetch(imgUrl)
+    if (!res.ok) throw new Error('下载失败')
+    const blob = await res.blob()
+    _downloadBlob(blob, downloadFilename.value)
     ElMessage.success('下载成功')
-  }).catch(() => {
-    ElMessage.error('下载失败，请重试')
-  }).finally(() => {
+  } catch {
+    // 远程 URL fetch 失败时新窗口打开
+    if (imgUrl.startsWith('http')) {
+      window.open(imgUrl, '_blank')
+    } else {
+      ElMessage.error('下载失败，请重试')
+    }
+  } finally {
     downloading.value = false
-  })
+  }
 }
 
 // 切换视角时重置缩放