feat: error page + navbar active styling (v1.18.0)

- Add SSO-style error page (views/error.ejs) and render it for browser
  navigation in the error handler (API still returns JSON).
- Navbar: username not underlined; only the active nav link is bold+underlined.

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

nodejs/public/css/styles.css

@@ -3,6 +3,12 @@ nav.navbar{
 	padding-right: 1em;
 }
 
+/* Only the active top-nav link is bold + underlined; the username is plain. */
+.top-nav a.active{
+	font-weight: bold;
+	text-decoration: underline;
+}
+
 body {
 	display: flex;
 	flex-direction: column;