feat(scenario): shared browser per run, sign step type, key loading helper

- all steps in a run share one browser/page instance so wizard state persists
- add 'sign' step type: execCode is {keyId} and signs via EDS widget on current page
- extract AuthService.loadKeyDescriptor() to deduplicate key file parsing
- extend StepType, DTOs, and MCP enums to include 'sign'

src/scenario/scenario-step.entity.ts

@@ -9,7 +9,7 @@ import {
 } from 'typeorm';
 import { ScenarioEntity } from './scenario.entity';
 
-export type StepType = 'login' | 'exec';
+export type StepType = 'login' | 'exec' | 'sign';
 
 @Entity('scenario_steps')
 export class ScenarioStepEntity {