- Add PluginStoreSettingsPage with header, language filters, and search
- Add PluginStoreSettingsViewModel with async loading and filtering
- Add PluginStoreItemViewModel for individual plugin cards with install/update/uninstall
- Connect AvaloniaPublicAPI to real PluginsManifest instead of empty stubs
- Use FluentAvalonia ItemsRepeater with UniformGridLayout for virtualization
- Fix icon visibility using ObjectConverters instead of StringConverters