fix: simplificar navbar, iconos siempre visibles

- Quitar AnimatedSwitcher/RotationTransition que ocultaba los iconos
- Icono sol/luna y campana con InkWell simple y colores explicitos
- Fondo navbar animado segun tema (isDark)
- DashboardLayout usa scaffoldBackgroundColor del tema

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

lib/ui/layouts/dashboard/dashboard_layout.dart

@@ -27,7 +27,7 @@ class _DashboardLayoutState extends State<DashboardLayout>
     final size = MediaQuery.of(context).size;
 
     return Scaffold(
-      backgroundColor: const Color(0xffEDF1F2),
+      backgroundColor: Theme.of(context).scaffoldBackgroundColor,
       body: Stack(
         children: [
           Row(