feat: app overhaul — Briefing-first navigation, Library, capture queue

Plans 1-3 implemented:

Plan 1 — Foundation
- Add google_fonts ^6.2.1
- lib/core/theme.dart: slate-indigo ColorScheme (dark/light), Fraunces
  headings, GradientButton widget
- lib/providers/capture_work_queue_provider.dart: in-memory sequential
  work queue; CaptureWorkQueueNotifier drains one item at a time;
  captureResultProvider feeds snackbars to UI
- lib/app.dart: wire fabledDarkTheme/fabledLightTheme; replace blocking
  _QuickCaptureBar with queue-based implementation (progress bar, badge)

Plan 2 — Navigation
- 3-tab shell: Briefing · Library · Chat (was Notes · Tasks · Projects · Chat)
- lib/screens/library/library_screen.dart: unified notes+tasks+projects list
  with filter pills, status sub-filter for tasks, live search, FAB
- lib/widgets/library_item_card.dart: NoteLibraryCard, TaskLibraryCard
  (status cycle), ProjectLibraryCard
- lib/screens/chat/conversations_tab_screen.dart: focused replacement for
  ConversationsListScreen
- Delete 5 dead screens: notes_list, tasks_list, project_list,
  conversations_list, quick_capture

Plan 3 — Briefing
- lib/data/models/briefing_conversation.dart
- lib/data/api/briefing_api.dart: getToday, getHistory, getMessages,
  triggerSlot
- lib/providers/briefing_provider.dart: BriefingNotifier with sendReply
  (optimistic + SSE + poll, same pattern as MessagesNotifier) and refresh
- lib/widgets/chat_message_bubble.dart: extracted + redesigned shared bubble
  (ghost user bubbles, left-accent assistant bubbles)
- lib/widgets/briefing_digest_card.dart: expandable first-message card
- lib/screens/briefing/briefing_screen.dart: digest card, conversation list,
  streaming reply bar, refresh button, history overflow menu
- lib/screens/briefing/briefing_history_screen.dart: read-only past dates

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

lib/providers/api_client_provider.dart

@@ -4,6 +4,7 @@ import 'package:flutter_riverpod/flutter_riverpod.dart';
 
 import '../data/api/api_client.dart';
 import '../data/api/auth_api.dart';
+import '../data/api/briefing_api.dart';
 import '../data/api/chat_api.dart';
 import '../data/api/milestones_api.dart';
 import '../data/api/notes_api.dart';
@@ -80,3 +81,7 @@ final milestonesApiProvider = Provider<MilestonesApi>((ref) {
 final milestonesRepositoryProvider = Provider<MilestonesRepository>((ref) {
   return MilestonesRepository(ref.watch(milestonesApiProvider));
 });
+
+final briefingApiProvider = Provider<BriefingApi>((ref) {
+  return BriefingApi(ref.watch(dioProvider));
+});