feat(plugins): show installed state in marketplace and stream install tasks

- report download progress (bytes, speed) and human-readable install stages
  from the plugin runtime connector via the install task context
- add a marketplace installed-state helper so market cards reflect whether a
  plugin is already installed
- surface install progress / queue state in the plugin install-task UI
- extend BackendClient and API entities with install-task status
- i18n for the new marketplace / install strings

web/src/i18n/locales/en-US.ts

@@ -748,6 +748,9 @@ const enUS = {
       'Are you sure you want to install plugin "{{name}}" ({{version}})?',
     downloadComplete: 'Plugin "{{name}}" download completed',
     installFailed: 'Installation failed, please try again later',
+    installed: 'Installed',
+    updateAvailable: 'Update available',
+    alreadyInstalled: '{{name}} is already installed',
     loadFailed: 'Failed to get plugin list, please try again later',
     noDescription: 'No description available',
     recommendation: {