feat(journal): replace briefing surface with journal; remove news/RSS

The backend retired /api/briefing/* and the RSS feature entirely. This
Flutter change mirrors what landed web-side: rename the briefing surface
to journal, repoint at /api/journal/*, and drop the news/RSS UI since
its endpoints no longer exist.

New (mirrors briefing structure with adapted shapes):
- lib/data/api/journal_api.dart — getToday, getDay, getDays, triggerPrep
- lib/data/models/journal_day.dart — {day_date, conversation, messages}
- lib/providers/journal_provider.dart — async notifier, sendReply, polling,
  silent refresh, regeneratePrep. Mirrors the briefing notifier 1:1
- lib/widgets/journal_prep_card.dart — adapted briefing_digest_card
- lib/screens/journal/journal_screen.dart — adapted briefing_screen,
  weather card preserved (rendered from msg_metadata.sections.weather
  on the daily-prep assistant message). News cards / RSS reactions /
  article-discuss removed
- lib/screens/journal/journal_history_screen.dart — past days picker
  pulls /api/journal/days, drills into /api/journal/day/<iso>

Wiring:
- Routes.briefing → Routes.journal (constants.dart)
- Routes.news removed
- briefingApiProvider → journalApiProvider (api_client_provider.dart)
- newsApiProvider removed
- app.dart: shell tab "Briefing" → "Journal"; News destination removed
  from nav rail, bottom nav, and the More sheet
- splash_screen.dart and login_screen.dart: redirect Routes.journal
  instead of Routes.briefing
- chat_api.dart: drop openArticleInChat (calls deleted /api/chat/from-article)
- settings_provider.dart: drop rssEnabled getter and rssEnabledProvider

Deleted:
- lib/screens/briefing/ (whole directory)
- lib/screens/news/ (whole directory)
- lib/data/api/briefing_api.dart, news_api.dart
- lib/data/models/briefing_conversation.dart, briefing_feed.dart, news_item.dart
- lib/providers/briefing_provider.dart, news_provider.dart
- lib/widgets/briefing_digest_card.dart, news_card.dart
- test cases for NewsItem and BriefingFeed in test/widget_test.dart

flutter analyze: 0 issues.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

lib/core/constants.dart

@@ -16,8 +16,7 @@ abstract class Routes {
   static const chat = '/chat/:id';
   static const quickCapture = '/quick-capture';
   static const settings = '/settings';
-  static const briefing = '/briefing';
-  static const news = '/news';
+  static const journal = '/journal';
   static const calendar = '/calendar';
   static const projectTasks = '/projects/:id/tasks';
 }