feat: optional worker selection when creating workflow (API + frontend dropdown)

src/api/workflows.rs

@@ -25,6 +25,8 @@ pub struct CreateWorkflow {
     pub requirement: String,
     #[serde(default)]
     pub template_id: Option<String>,
+    #[serde(default)]
+    pub worker: Option<String>,
 }
 
 #[derive(Deserialize)]
@@ -78,6 +80,7 @@ async fn create_workflow(
         workflow_id: workflow.id.clone(),
         requirement: workflow.requirement.clone(),
         template_id: input.template_id,
+        worker: input.worker,
     }).await;
 
     Ok(Json(workflow))