fix(海报生成): 修复iOS微信中海报生成失败和封面空白问题

- 添加隐藏的预渲染节点，避免动态计算容器宽度导致的封面空白
- 在iOS微信环境中使用html2canvas替代html-to-image以解决兼容性问题
- 优化封面图片加载逻辑，添加原图兜底策略
- 调整样式和监听逻辑，确保数据变化时能正确触发重新生成

@@ -24,7 +24,7 @@
                 <div v-else>
                     <!-- 上部封面图 -->
                     <div class="PosterCover rounded-t-xl overflow-hidden">
-                        <img :src="cover_final_src" alt="课程封面" class="w-full h-auto object-contain" crossorigin="anonymous" data-role="cover" />
+                        <img :src="cover_final_src" alt="课程封面" class="w-full h-auto object-contain" crossorigin="anonymous" data-role="cover" loading="eager" />
                     </div>
                 <!-- 下部信息区：左二维码 + 右文案 -->
                 <div class="PosterInfo p-4">
@@ -34,11 +34,11 @@
                                 <img :src="qr_final_src" alt="课程二维码" class="w-24 h-24 rounded" crossorigin="anonymous" />
                             </div>
                             <!-- 右侧文案 -->
-                            <div class="flex-1 flex flex-col space-y-2 -mt-1">
-                                <div class="text-lg font-semibold text-gray-800 truncate leading-tight -mt-0.5">{{ title_text }}</div>
-                                <div class="text-sm text-gray-500 truncate" v-if="subtitle_text">{{ subtitle_text }}</div>
-                                <div class="text-sm text-gray-400 truncate" v-if="date_range_text">{{ date_range_text }}</div>
-                                <div class="mt-2 text-green-600 text-sm">扫码了解详情</div>
+                            <div class="flex-1 flex flex-col space-y-1">
+                                <div class="text-lg font-semibold text-gray-800 leading-normal">{{ title_text }}</div>
+                                <div class="text-sm text-gray-500 leading-normal" v-if="subtitle_text">{{ subtitle_text }}</div>
+                                <div class="text-sm text-gray-400 leading-normal" v-if="date_range_text">{{ date_range_text }}</div>
+                                <div class="mt-2 text-green-600 text-sm leading-normal">扫码了解详情</div>
                             </div>
                         </div>
                     </div>
@@ -52,12 +52,37 @@
             </div>
         </div>
     </van-popup>
+    <div class="fixed inset-0 opacity-[0.01] pointer-events-none -z-10">
+        <div class="PosterWrapper p-4 w-screen">
+            <div class="PosterCard bg-white rounded-xl overflow-hidden border border-gray-200 mx-auto" ref="preload_ref">
+                <div>
+                    <div class="PosterCover rounded-t-xl overflow-hidden">
+                        <img :src="cover_final_src" alt="课程封面" class="w-full h-auto object-contain" crossorigin="anonymous" data-role="cover" loading="eager" />
+                    </div>
+                    <div class="PosterInfo p-4">
+                        <div class="flex items-start">
+                            <div class="PosterQR mr-4">
+                                <img :src="qr_final_src" alt="课程二维码" class="w-24 h-24 rounded" crossorigin="anonymous" />
+                            </div>
+                            <div class="flex-1 flex flex-col space-y-1">
+                                <div class="text-lg font-semibold text-gray-800 leading-normal">{{ title_text }}</div>
+                                <div class="text-sm text-gray-500 leading-normal" v-if="subtitle_text">{{ subtitle_text }}</div>
+                                <div class="text-sm text-gray-400 leading-normal" v-if="date_range_text">{{ date_range_text }}</div>
+                                <div class="mt-2 text-green-600 text-sm leading-normal">扫码了解详情</div>
+                            </div>
+                        </div>
+                    </div>
+                </div>
+            </div>
+        </div>
+    </div>
     <van-back-top right="5vw" bottom="25vh" offset="600" />
 </template>
 
 <script setup>
 import { ref, computed, watch, nextTick } from 'vue'
 import { toPng } from 'html-to-image'
+import html2canvas from 'html2canvas'
 import QRCode from 'qrcode'
 import { showToast } from 'vant'
 import { MIN_TIMEOUT, DEFAULT_FETCH_TIMEOUT } from '@/common/constants'
@@ -158,6 +183,7 @@ const show_proxy = computed({
  * @description 海报卡片容器引用，用于动态获取容器宽度以计算 Canvas 尺寸
  */
 const card_ref = ref(null)
+const preload_ref = ref(null)
 
 /**
  * @var {import('vue').Ref<boolean>} is_generating
@@ -302,7 +328,7 @@ async function prepare_assets() {
     try {
         const raw = props.course?.cover || ''
         // 依据容器宽度与像素比计算目标宽度，设置上限避免过大导致内存/渲染失败
-        const base_w = Math.max(320, Math.round((card_ref.value?.clientWidth || 750)))
+        const base_w = Math.max(320, Math.round((preload_ref.value?.clientWidth || card_ref.value?.clientWidth || 750)))
         const dpr = Math.max(1, Math.min(2, window.devicePixelRatio || 1))
         const target_w = Math.min(900, Math.round(base_w * dpr))
         const big_url = build_cdn_thumbnail(raw, target_w)
@@ -315,6 +341,11 @@ async function prepare_assets() {
             const b64_small = await fetch_to_data_url_with_timeout(small_url, 2000)
             cover_data_url.value = b64_small || ''
         }
+        // 封面兜底：使用原图地址再尝试一次转 base64，减少移动端空白
+        if (!cover_data_url.value && raw) {
+            const b64_origin = await fetch_to_data_url_with_timeout(raw, 3500)
+            cover_data_url.value = b64_origin || ''
+        }
     } catch (e) {
         cover_data_url.value = ''
     }
@@ -326,14 +357,13 @@ async function prepare_assets() {
  * @returns {Promise<void>}
  */
 async function compose_poster() {
-    poster_img_src.value = ''
     try {
         // 标记进入生成流程
         is_generating.value = true
         // 准备资源，尽量使用 base64 避免跨域失败
         await prepare_assets()
         await nextTick()
-        const node = card_ref.value
+        const node = preload_ref.value || card_ref.value
         if (!node) {
             is_generating.value = false
             return
@@ -349,25 +379,39 @@ async function compose_poster() {
         const pixel_ratio = Math.max(1, Math.min(2.5, window.devicePixelRatio || 1))
         // 透明 1x1 PNG 作为占位图，避免资源获取失败直接中断
         const placeholder = 'data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAQAAAC1HAwCAAAAC0lEQVR42mP8/x8AAusB9Wm3T9kAAAAASUVORK5CYII='
-        // 生成 PNG dataURL
-        const data_url = await toPng(clone, {
-            pixelRatio: pixel_ratio,
-            cacheBust: true,
-            imagePlaceholder: placeholder,
-            fetchRequestInit: { mode: 'cors', cache: 'no-cache', credentials: 'omit' },
-            // 避免外层 margin 影响截图结果
-            style: { margin: '0' },
-            width: Math.round(size.width),
-            height: Math.round(size.height)
-        })
+        const ua = typeof navigator !== 'undefined' ? navigator.userAgent : ''
+        const is_ios_wechat = /MicroMessenger/i.test(ua) && /iPhone|iPad|iPod/i.test(ua)
+        let data_url = ''
+        if (is_ios_wechat) {
+            const canvas = await html2canvas(clone, {
+                scale: pixel_ratio,
+                useCORS: true,
+                allowTaint: false,
+                backgroundColor: null,
+                width: Math.round(size.width),
+                height: Math.round(size.height)
+            })
+            data_url = canvas.toDataURL('image/png')
+        } else {
+            data_url = await toPng(clone, {
+                pixelRatio: pixel_ratio,
+                cacheBust: true,
+                imagePlaceholder: placeholder,
+                fetchRequestInit: { mode: 'cors', cache: 'no-cache', credentials: 'omit' },
+                // 避免外层 margin 影响截图结果
+                style: { margin: '0' },
+                width: Math.round(size.width),
+                height: Math.round(size.height)
+            })
+        }
         poster_img_src.value = data_url
+        has_generated_once.value = true
         // 清理离屏节点
         if (wrapper && wrapper.parentNode) wrapper.parentNode.removeChild(wrapper)
         is_generating.value = false
     } catch (err) {
         console.error('html-to-image 生成海报失败:', err)
         showToast('海报生成失败，已展示标准卡片，请长按保存截图')
-        poster_img_src.value = ''
         is_generating.value = false
     }
 }
@@ -379,15 +423,7 @@ async function compose_poster() {
  */
 async function generate_on_open() {
     try {
-        if (!has_generated_once.value) {
-            await compose_poster()
-            // 短暂等待，确保图片与布局稳定后再次生成，规避首次封面空白
-            await new Promise(r => setTimeout(r, 120))
-            await compose_poster()
-            has_generated_once.value = true
-        } else {
-            await compose_poster()
-        }
+        await compose_poster()
     } catch (_) {
         // 忽略异常，compose_poster 已包含错误提示
     }
@@ -401,7 +437,8 @@ async function generate_on_open() {
  */
 function create_offscreen_clone(node) {
     const rect = node.getBoundingClientRect()
-    const size = { width: rect.width, height: rect.height }
+    const extra = 12
+    const size = { width: rect.width, height: rect.height + extra }
     const wrapper = document.createElement('div')
     const clone = node.cloneNode(true)
     // 离屏包裹容器样式
@@ -422,6 +459,7 @@ function create_offscreen_clone(node) {
         width: '100%',
         height: '100%',
         margin: '0',
+        paddingBottom: `${extra}px`,
         transform: 'none',
         filter: 'none',
         boxShadow: 'none',
@@ -440,8 +478,8 @@ function create_offscreen_clone(node) {
  */
 watch(show_proxy, (opened) => {
     if (opened) {
+        if (poster_img_src.value) return
         if (is_generating.value) return
-        poster_img_src.value = ''
         nextTick(() => generate_on_open())
     }
 })
@@ -506,13 +544,26 @@ async function fetch_to_data_url_with_timeout(url, timeout_ms) {
 }
 
 // 调整数据变更监听：仅在封面地址或二维码地址变化时重新生成
-watch([() => props.course?.cover, () => props.qr_url], () => {
-    if (show_proxy.value) {
-        if (is_generating.value) return
-        poster_img_src.value = ''
-        nextTick(() => compose_poster())
-    }
+const poster_source_list = computed(() => [
+    props.course?.cover || '',
+    props.course?.title || '',
+    props.course?.subtitle || '',
+    props.course?.course_start_time || props.course?.start_at || '',
+    props.course?.course_end_time || props.course?.end_at || '',
+    props.qr_url || ''
+])
+
+const is_poster_ready = computed(() => {
+    const title_value = String(props.course?.title || '').trim()
+    const cover_value = String(props.course?.cover || '').trim()
+    return Boolean(title_value || cover_value)
 })
+
+watch(poster_source_list, () => {
+    if (!is_poster_ready.value) return
+    if (is_generating.value) return
+    nextTick(() => compose_poster())
+}, { immediate: true })
 </script>
 
 <style lang="less" scoped>
@@ -540,13 +591,14 @@ watch([() => props.course?.cover, () => props.qr_url], () => {
                 overflow: hidden;
             }
             .PosterInfo {
-                overflow: hidden;
+                overflow: visible;
+                padding-bottom: 8px;
                 .PosterQR {
-                img {
-                    box-shadow: none;
+                    img {
+                        box-shadow: none;
+                    }
                 }
             }
         }
     }
-}
 </style>