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/views/top.ejs

@@ -49,7 +49,7 @@
 				</ul>
 				<div class="form-inline mt-2 mt-md-0">
 					<% if(ui.profileUrl){ %>
-					<a id="cl-username" class="navbar-text text-light me-3" href="<%- ui.profileUrl %>" style="display: none;">
+					<a id="cl-username" class="navbar-text text-light me-3 text-decoration-none" href="<%- ui.profileUrl %>" style="display: none;">
 						<i class="fa-solid fa-user me-1"></i><span id="cl-username-text"></span>
 					</a>
 					<% } else { %>