* 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.