Release 1.1.18: fix Sites page crash, refresh screenshots

The Sites & Replication page (added in the prior multi-master LDAP
release) 500'd on every load: views/sites.ejs included nonexistent
partials 'header'/'footer' instead of this app's actual 'top'/'bottom'.
Fixed to match every other view.

Also refreshed all README screenshots against the current UI and added
a new Sites & Replication screenshot.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

@@ -25,6 +25,10 @@ phone-home, no hosted control plane, and no per-user pricing.
 | --- | --- |
 | [![Groups](docs/images/groups.png)](docs/images/groups.png) | [![OAuth clients](docs/images/oauth-clients.png)](docs/images/oauth-clients.png) |
 
+| Sites & Replication |
+| --- |
+| [![Sites](docs/images/sites.png)](docs/images/sites.png) |
+
 ## Features
 
 - **OpenID Connect / OAuth 2.0 provider** — issue your own access, refresh, and