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/create-scenario-step.dto.ts

@@ -24,14 +24,14 @@ export class CreateScenarioStepDto {
   @IsIn(["login", "exec", "sign"])
   type: StepType;
 
-  @ApiProperty({
-    description:
-      "Session name used by this step. login steps create it; exec steps consume it.",
+  @ApiPropertyOptional({
+    description: "Session name (deprecated — browser is created automatically per run).",
     example: "my-session",
   })
+  @IsOptional()
   @IsString()
   @IsNotEmpty()
-  sessionName: string;
+  sessionName?: string;
 
   @ApiPropertyOptional({ example: "return await page.title();" })
   @IsOptional()