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/app/infra/entities/api/index.ts

@@ -460,6 +460,8 @@ export interface AsyncTask {
   name: string;
   label: string;
   task_type: string; // system or user
+  /** Unix epoch seconds (float) when the task was created. */
+  created_at?: number;
   runtime: AsyncTaskRuntimeInfo;
   task_context: AsyncTaskTaskContext;
 }