fix: Add navigation styles to dashboards

- Add mode navigation CSS to core/layout.css so it's available on all pages
- Import layout.css in dashboard stylesheets (ADSB, AIS, Satellite)
- Fixes nav partial appearing unstyled on dashboard pages

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

static/css/adsb_dashboard.css

@@ -3,6 +3,7 @@
  * Imports design tokens for consistency, with dashboard-specific overrides
  */
 @import url('./core/variables.css');
+@import url('./core/layout.css');
 
 * {
     margin: 0;