* feat(ui): add Rescan button to plugin list empty state
When no plugins are installed and the folder watcher fails to detect new
plugins, users had no way to trigger a rescan. Extract RescanButton into
a shared component and render it in a custom empty state for the plugin
list.
* refactor(ui): address review feedback for plugin empty state
- Pass label translation key directly to RA Button (auto-translates)
- Use within() from Testing Library instead of querySelector for
scoped queries with better error messages
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