feat: add news ticker widget for scrolling headlines

- Add NewsTickerWidget with horizontal auto-scrolling at 40px/sec
- Add NewsData and NewsHeadline Freezed models
- Add news datasource fetching from /tools/news endpoint
- Add news provider with 30-minute auto-refresh
- Place ticker between Welcome card and System Stats on dashboard
- Show placeholder headlines when no data (italic, muted style)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

lib/shared/widgets/widgets.dart

@@ -4,4 +4,5 @@ library;
 export 'air_quality_widget.dart';
 export 'gauge_widget.dart';
 export 'icon_picker.dart';
+export 'news_ticker_widget.dart';
 export 'weather_widget.dart';