Fix nested nav bar appearing in embedded dashboard iframes

When dashboards (satellite, ADS-B, AIS) are loaded via iframe with
?embedded=true, the full navigation bar was still rendered, creating
a "UI in UI" effect. Pass the embedded query param from route handlers
to templates and conditionally skip the nav include.

Fixes #144

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

templates/layout/base_dashboard.html

@@ -166,7 +166,9 @@
 
 {% block navigation %}
 {# Include the unified nav partial with active_mode set #}
+{% if not embedded %}
 {% include 'partials/nav.html' with context %}
+{% endif %}
 {% endblock %}
 
 {% block main %}