Prevent duplicate/plaintext login announcement rendering (#6963)

* Initial plan

* Fix duplicate login announcement display

Co-authored-by: DerLinkman <62480600+DerLinkman@users.noreply.github.com>

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: DerLinkman <62480600+DerLinkman@users.noreply.github.com>

data/web/templates/base.twig

@@ -144,7 +144,7 @@
 
 <form action="/" method="post" id="logout"><input type="hidden" name="logout"></form>
 
-{% if ui_texts.ui_announcement_text and ui_texts.ui_announcement_active and not is_root_uri %}
+{% if ui_texts.ui_announcement_text and ui_texts.ui_announcement_active and not is_root_uri and mailcow_cc_username %}
 <div class="container mt-4">
   <div class="alert alert-{{ ui_texts.ui_announcement_type }}">{{ ui_texts.ui_announcement_text }}</div>
 </div>