feat(snippets): add snippets entity, crud, and auto-browser-per-run

- add Snippet entity with name, description, code; full CRUD backend
- add snippets pages (list, create, edit, detail) and nav entry
- add runSnippet helper in code-executor using new Function with args array
- add result param to execute() so validateCode can access exec output
- remove sessionName from steps; each run now spawns its own fresh browser
- fix waitForURL race by polling localStorage for token instead

server/src/scenario/dto/scenario-export.dto.ts

@@ -25,7 +25,7 @@ export class ScenarioStepExportDto {
   @ApiProperty()
   @IsString()
   @IsNotEmpty()
-  sessionName: string;
+  sessionName: string | null;
 
   @ApiPropertyOptional()
   @IsOptional()