Add logout button to header

Adds a logout button in the top bar on authenticated pages. Posts to
/api/auth/logout to clear the server session, then redirects to login.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/static/js/app.js

@@ -9,6 +9,7 @@ const NextSnap = {
         console.log(`NextSnap v${this.version} initializing...`);
         this.setupConnectivityMonitoring();
         this.setupServiceWorkerMessaging();
+        this.setupLogout();
         this.checkHealth();
     },
 
@@ -79,6 +80,17 @@ const NextSnap = {
         }
     },
 
+
+    setupLogout() {
+        const btn = document.getElementById('logoutBtn');
+        if (!btn) return;
+        btn.addEventListener('click', async () => {
+            try {
+                await fetch('/api/auth/logout', { method: 'POST' });
+            } catch (e) { /* offline - clear locally anyway */ }
+            window.location.href = '/login';
+        });
+    },
     // Force service worker update
     async updateServiceWorker() {
         if ('serviceWorker' in navigator) {