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>

routes/ais.py

@@ -540,7 +540,9 @@ def get_vessel_dsc(mmsi: str):
 @ais_bp.route('/dashboard')
 def ais_dashboard():
     """Popout AIS dashboard."""
+    embedded = request.args.get('embedded', 'false') == 'true'
     return render_template(
         'ais_dashboard.html',
         shared_observer_location=SHARED_OBSERVER_LOCATION_ENABLED,
+        embedded=embedded,
     )