feat(web): 桌面端前端 Web 独立部署适配——浏览器降级桥+登录态引导+页面动态修正

- 新增 web-fallback.ts：无 pywebview 时以浏览器能力降级（文件选择/上传/模板下载/配置），
  其余桌面能力（enqueue_json/vc_*/folder 选择等）保持 undefined 交由页面既有请在本地客户端中打开提示
- 新增 ensure-auth.ts：无 uid 时经 /newApi/check_login 恢复登录态，无 token/失效跳登录页；
  21 个入口 main.ts 挂载前先引导（桌面端已注入 uid 时零开销）
- pywebview.ts：getPywebviewApi() 在纯浏览器环境返回降级桥；新增 isDesktopRuntime 标志
  （首页更新面板/退出链接据此区分桌面与 Web）；hasPywebview 语义修正为真实桥判定
- 登录页跳转、首页登出链接、品牌检测页 hasBridge 判定（select_brand_folder 存在性）适配 Web

frontend-vue/src/pages/image-video/ImageVideoPage.vue

@@ -2,7 +2,7 @@
   <div v-if="currentView === 'menu'" class="image-video-page">
     <header class="page-header">
       <span class="header-title">数富AI</span>
-      <a class="back-link" href="/home">返回首页</a>
+      <a class="back-link" :href="goHome()">返回首页</a>
     </header>
 
     <main class="entrances" :class="{ 'is-loading': permissionsLoading }" aria-label="视频与图片入口">
@@ -21,9 +21,12 @@
       <button v-if="hasMenuPermission('mix-video')" type="button" class="entrance-btn" @click="showSoon('混剪')">
         混剪
       </button>
-      <a v-if="hasMenuPermission('image')" class="entrance-btn" href="/image">
+      <a v-if="hasMenuPermission('image') && isDesktop" class="entrance-btn" href="/new_web_source/image.html">
         图片
       </a>
+      <button v-else-if="hasMenuPermission('image')" type="button" class="entrance-btn" @click="showDesktopOnly('图片工作台')">
+        图片
+      </button>
     </main>
 
     <div class="toast" :class="{ show: Boolean(statusText), error: statusType === 'error' }">
@@ -75,10 +78,22 @@ import BrandTopBar from '@/pages/brand/components/BrandTopBar.vue'
 import DeliveryVideoWorkspace from '@/pages/image-video/components/DeliveryVideoWorkspace.vue'
 import { getCurrentUserAppColumnKeys } from '@/shared/api/permission'
 import { getPywebviewApi } from '@/shared/bridges/pywebview'
+import { isDesktopClientPage } from '@/shared/page-prefix'
 import DownloadProgressPanel from '@/shared/components/DownloadProgressPanel.vue'
 
 type ViewMode = 'menu' | 'delivery'
 
+const isDesktop = isDesktopClientPage()
+
+function goHome() {
+  // 桌面端首页是 Flask /home（重定向到 Vue home）；dev 下是 MPA 的 /home.html
+  return isDesktop ? '/home' : '/home.html'
+}
+
+function showDesktopOnly(name: string) {
+  showStatus(`${name}为桌面客户端功能，请在本地客户端中打开`, 'error')
+}
+
 const currentView = ref<ViewMode>('menu')
 const allowedColumnKeys = ref(new Set<string>())
 const permissionsLoading = ref(true)
@@ -184,7 +199,7 @@ onBeforeUnmount(() => {
 async function launchDesktop() {
   const api = getPywebviewApi()
   if (!api?.launch_yaoayanui) {
-    showStatus('当前环境不支持拉起桌面端', 'error')
+    showStatus('数字人为桌面客户端程序：请在本机客户端中打开后再点击', 'error')
     return
   }
 
@@ -246,7 +261,7 @@ async function launchDesktop() {
   font-family: "Microsoft YaHei", "PingFang SC", "Hiragino Sans GB", sans-serif;
   background:
     linear-gradient(rgba(255, 255, 255, 0.5), rgba(255, 255, 255, 0.5)),
-    url('/static/bg.jpg') center/cover no-repeat,
+    url('/bg.jpg') center/cover no-repeat,
     #eef3f7;
 }