catalog: add dsh-video-preview (video previewer) to the file-previewer recommended plugins (#126)

* catalog: add dsh-video-preview to the file-previewer recommended plugins

Adds the video preview plugin (dsh-video-preview) to the "add preview plugin" modal catalog, with zh/en descriptions. Data integrity is guarded by tests/plugin-list.spec.ts.

* test: make add-plugin-modal window.open count catalog-driven

The test pinned the viewer catalog to a single entry (2 "Open:" buttons).
Adding a second recommended plugin (dsh-video-preview) renders 4 buttons
and broke the hardcoded assertion. Derive the expected count from
builtinViewerPlugins so adding/removing plugins keeps the test honest.

src/client/plugins-viewers.ts

@@ -20,4 +20,11 @@ export const builtinViewerPlugins: readonly PluginEntry[] = [
     description: () => t('pluginOfficeDesc'),
     install: 'cd ~/.dsh && dsh plugin --profile web add @huanlin/dsh-plugin-better-sidebar-plugin-office',
   },
+  {
+    id: 'dsh-video-preview',
+    name: '视频预览插件',
+    url: 'https://github.com/zemul/dsh-video-preview',
+    description: () => t('pluginVideoPreviewDesc'),
+    install: 'cd ~/.dsh && dsh plugin --profile web add dsh-video-preview',
+  },
 ]