Add HAProxy monitoring dashboard features

- Introduced a new `dashboard` frontend in HAProxy configuration to serve a real-time monitoring dashboard.
- Added support for returning prebuilt `dashboard.html` for `/` or `/index.html` paths.
- Updated test cases and expected outputs to accommodate the new frontend.

tests/expected/services-letsencrypt.txt

@@ -45,6 +45,14 @@ backend srv_stats
     mode http
     server Local 127.0.0.1:1936
 
+frontend dashboard
+    bind *:11936
+    mode http
+    acl is_index path /
+    acl is_index path /index.html
+    http-request return status 200 content-type "text/html" file /etc/easyhaproxy/www/dashboard.html if is_index
+    http-request return status 404
+
 frontend http_in_80
     bind *:80
     mode http