fix(ui): 修复 Banner 封面图不显示

MarkdownViewer 的 baseURL 去掉了 /api 前缀，导致
image_proxy 请求路径错误。改为组件内部直接读取
VITE_API_BASE_URL，与 NoteHistory 保持一致。

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

BillNote_frontend/src/pages/HomePage/components/MarkdownViewer.tsx

@@ -233,7 +233,6 @@ const MarkdownViewer: FC<MarkdownViewerProps> = ({ status }) => {
                   <VideoBanner
                     audioMeta={currentTask?.audioMeta}
                     videoUrl={currentTask?.formData?.video_url}
-                    baseURL={baseURL}
                   />
                 </div>
                 <div className={'markdown-body w-full px-2'}>

BillNote_frontend/src/pages/HomePage/components/VideoBanner.tsx

@@ -4,7 +4,6 @@ import type { AudioMeta } from '@/store/taskStore'
 interface VideoBannerProps {
   audioMeta?: AudioMeta
   videoUrl?: string
-  baseURL?: string
 }
 
 /** 平台 label 映射 */
@@ -15,13 +14,14 @@ const platformLabel: Record<string, string> = {
   xiaohongshu: '小红书',
 }
 
-export default function VideoBanner({ audioMeta, videoUrl, baseURL = '' }: VideoBannerProps) {
+export default function VideoBanner({ audioMeta, videoUrl }: VideoBannerProps) {
   if (!audioMeta) return null
 
   const rawCover = audioMeta.cover_url
   // 通过后端代理加载封面，避免跨域/Referrer 限制
+  const apiBase = String(import.meta.env.VITE_API_BASE_URL || 'api').replace(/\/$/, '')
   const coverUrl = rawCover
-    ? `${baseURL}/image_proxy?url=${encodeURIComponent(rawCover)}`
+    ? `${apiBase}/image_proxy?url=${encodeURIComponent(rawCover)}`
     : ''
   const title = audioMeta.title
   const uploader = audioMeta.raw_info?.uploader || ''