Improve security, accessibility, and performance

- Add Content Security Policy meta tag for XSS protection
- Add width/height to image for CLS prevention
- Add aria-hidden to decorative SVG icons (7 locations)
- Refactor inline onclick to addEventListener for CSP compliance
- Add visibility-aware setInterval for ping updates
- Add keyboard navigation (Arrow keys) for tab panels

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

app.js

@@ -323,5 +323,63 @@ function updatePing() {
   });
 }
 
-// Ping 업데이트 시작
-setInterval(updatePing, 2000);
+// Ping 업데이트 시작 (visibility-aware)
+let pingInterval;
+
+function startPingUpdates() {
+  if (!pingInterval) {
+    pingInterval = setInterval(updatePing, 2000);
+  }
+}
+
+function stopPingUpdates() {
+  if (pingInterval) {
+    clearInterval(pingInterval);
+    pingInterval = null;
+  }
+}
+
+// Visibility change handler
+document.addEventListener('visibilitychange', () => {
+  if (document.hidden) {
+    stopPingUpdates();
+  } else {
+    startPingUpdates();
+  }
+});
+
+// Initial start
+startPingUpdates();
+
+// Tab switching with event listeners and keyboard navigation
+document.addEventListener('DOMContentLoaded', () => {
+  const btnN8n = document.getElementById('btn-n8n');
+  const btnTf = document.getElementById('btn-tf');
+
+  if (btnN8n && btnTf) {
+    // Click event listeners
+    btnN8n.addEventListener('click', () => switchTab('n8n'));
+    btnTf.addEventListener('click', () => switchTab('tf'));
+
+    // Keyboard navigation (Arrow keys)
+    [btnN8n, btnTf].forEach(btn => {
+      btn.addEventListener('keydown', (e) => {
+        const currentTab = btn.getAttribute('data-tab');
+
+        if (e.key === 'ArrowRight') {
+          e.preventDefault();
+          if (currentTab === 'n8n') {
+            btnTf.focus();
+            switchTab('tf');
+          }
+        } else if (e.key === 'ArrowLeft') {
+          e.preventDefault();
+          if (currentTab === 'tf') {
+            btnN8n.focus();
+            switchTab('n8n');
+          }
+        }
+      });
+    });
+  }
+});