Fix medium priority issues from code review

- Add DEPLOY_TIMING constants for deployment simulation
- Add null checks in switchTab function for consistency
- Add role="radiogroup" and fieldset/legend to wizard steps
- Add role="log" and aria-live="polite" to chat container
- Add bottom border to active pricing region button for better visual distinction

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app.js

@@ -58,6 +58,14 @@ const PAYMENT_METHODS = [
   { id: 'yearly', name: '연간 결제', desc: '2개월 무료 (17% 할인)', discount: 17 }
 ];
 
+// 배포 시뮬레이션 타이밍 상수
+const DEPLOY_TIMING = {
+  IMAGE_READY: 1500,
+  CONTAINER_CREATED: 3000,
+  NETWORK_CONFIGURED: 4500,
+  COMPLETE: 6000
+};
+
 /**
  * 가격 포맷팅 (한국 원화)
  */
@@ -197,26 +205,26 @@ function anvilApp() {
         this.logs.push('✅ 이미지 준비 완료');
         this.logs.push('🔧 컨테이너 인스턴스 생성 중...');
         this.deployStep = 2;
-      }, 1500);
+      }, DEPLOY_TIMING.IMAGE_READY);
 
       setTimeout(() => {
         this.logs.push('✅ 컨테이너 생성 완료');
         this.logs.push('🌐 네트워크 및 방화벽 구성 중...');
         this.deployStep = 3;
-      }, 3000);
+      }, DEPLOY_TIMING.CONTAINER_CREATED);
 
       setTimeout(() => {
         const randomIP = Math.floor(Math.random() * 254 + 1);
         this.logs.push(`✅ IP 할당 완료: 45.12.89.${randomIP}`);
         this.logs.push('⚙️ 시스템 서비스 시작 중...');
         this.deployStep = 4;
-      }, 4500);
+      }, DEPLOY_TIMING.NETWORK_CONFIGURED);
 
       setTimeout(() => {
         this.launching = false;
         this.deployStep = 5;
         this.logs.push('🎉 서버가 활성화되었습니다!');
-      }, 6000);
+      }, DEPLOY_TIMING.COMPLETE);
     },
 
     // 런처 열기
@@ -275,6 +283,9 @@ function switchTab(tab) {
   const panelN8n = document.getElementById('panel-n8n');
   const panelTf = document.getElementById('panel-tf');
 
+  // Null checks for DOM elements
+  if (!btnN8n || !btnTf || !panelN8n || !panelTf) return;
+
   if (tab === 'n8n') {
     // Update ARIA states for n8n tab
     btnN8n.setAttribute('aria-selected', 'true');