Refactor websocket message type usage

Replaced string literals with WebsocketMessageType enum for agent messages and plan approval events in PlanPage and PlanDataService. This improves type safety and consistency when handling websocket events.

Author: Fr4nc3

src/frontend/src/pages/PlanPage.tsx

@@ -128,9 +128,9 @@ const PlanPage: React.FC = () => {
                 handleConnectionChange(message.data?.connected || false);
             });
 
-            const unsubscribeStreaming = webSocketService.on('agent_message', handleStreamingMessage);
-            const unsubscribePlanApproval = webSocketService.on('plan_approval_response', handlePlanApprovalResponse);
-            const unsubscribePlanApprovalRequest = webSocketService.on('plan_approval_request', handlePlanApprovalRequest);
+            const unsubscribeStreaming = webSocketService.on(WebsocketMessageType.AGENT_MESSAGE, handleStreamingMessage);
+            const unsubscribePlanApproval = webSocketService.on(WebsocketMessageType.PLAN_APPROVAL_RESPONSE, handlePlanApprovalResponse);
+            const unsubscribePlanApprovalRequest = webSocketService.on(WebsocketMessageType.PLAN_APPROVAL_REQUEST, handlePlanApprovalRequest);
             const unsubscribeParsedPlanApprovalRequest = webSocketService.on(WebsocketMessageType.PLAN_APPROVAL_REQUEST, handlePlanApprovalRequest);
 
             return () => {

src/frontend/src/services/PlanDataService.tsx

@@ -379,7 +379,7 @@ export class PlanDataService {
   } | null {
     try {
       // Unwrap wrapper
-      if (rawData && typeof rawData === 'object' && rawData.type === 'agent_message' && typeof rawData.data === 'string') {
+      if (rawData && typeof rawData === 'object' && rawData.type === WebsocketMessageType.AGENT_MESSAGE && typeof rawData.data === 'string') {
         return this.parseAgentMessage(rawData.data);
       }