feat(首页): 提升视频封面分辨率和质量

- 分辨率：200x → 800x
- 质量：70 → 90

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

@@ -228,7 +228,7 @@ const videoOptions = ref({
   aspectRatio: '16:9',
   controls: true,
   preload: 'metadata',
-  poster: 'https://cdn.ipadbiz.cn/stdj/video/workflow.jpg?imageMogr2/thumbnail/200x/strip/quality/70',
+  poster: 'https://cdn.ipadbiz.cn/stdj/video/workflow.jpg?imageMogr2/thumbnail/800x/strip/quality/90',
   playbackRates: [0.5, 1, 1.25, 1.5, 2],
 })