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/dto/create-scenario-step.dto.ts

@@ -8,8 +8,8 @@ export class CreateScenarioStepDto {
   @Min(0)
   order: number;
 
-  @ApiProperty({ enum: ['login', 'exec'], example: 'exec' })
-  @IsIn(['login', 'exec'])
+  @ApiProperty({ enum: ['login', 'exec', 'sign'], example: 'exec' })
+  @IsIn(['login', 'exec', 'sign'])
   type: StepType;
 
   @ApiProperty({ description: 'Session name used by this step. login steps create it; exec steps consume it.', example: 'my-session' })

src/scenario/dto/update-scenario-step.dto.ts

@@ -9,9 +9,9 @@ export class UpdateScenarioStepDto {
   @Min(0)
   order?: number;
 
-  @ApiPropertyOptional({ enum: ['login', 'exec'] })
+  @ApiPropertyOptional({ enum: ['login', 'exec', 'sign'] })
   @IsOptional()
-  @IsIn(['login', 'exec'])
+  @IsIn(['login', 'exec', 'sign'])
   type?: StepType;
 
   @ApiPropertyOptional()