feat: show connector error details for Plugin and Box runtime status

Record Box connector error in BoxService and expose it as
'connector_error' in GET /api/v1/box/status when unavailable.
Display error messages in the dashboard System Status popover
for both Plugin Runtime (plugin_connector_error) and Box Runtime
(connector_error) when they are disconnected.

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

@@ -355,6 +355,7 @@ export interface ApiRespBoxStatus {
   available: boolean;
   profile: string;
   recent_error_count: number;
+  connector_error?: string;
   backend?: {
     name: string;
     available: boolean;