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/css/style.css

@@ -773,3 +773,19 @@ select {
     color: var(--offline, #888);
     font-weight: 600;
 }
+
+/* Logout button in top bar */
+.logout-btn {
+    background: none;
+    border: 1px solid rgba(255, 255, 255, 0.2);
+    color: var(--text-secondary);
+    font-size: 0.75rem;
+    padding: 0.25rem 0.6rem;
+    border-radius: 4px;
+    cursor: pointer;
+    min-height: 28px;
+}
+
+.logout-btn:active {
+    opacity: 0.7;
+}