[ codeswot/nostrrdr ] Add adaptive filter screen with custom routing
Introduces a new FilterScreen with adaptive presentation as a modal bottom sheet on mobile and a dialog on larger screens. Adds AdaptiveRouteFactory for platform-adaptive route building, updates app_router to use a custom route for FilterScreen, and integrates the filter screen trigger in the home filter/search widget. Also enforces portrait orientation and edge-to-edge system UI in main.dart.