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/data/models/briefing_conversation.dart

@@ -0,0 +1,35 @@
+import 'message.dart';
+
+class BriefingConversation {
+  final int id;
+  final String title;
+  final String? briefingDate; // YYYY-MM-DD or null
+  final List<Message> messages;
+
+  const BriefingConversation({
+    required this.id,
+    required this.title,
+    this.briefingDate,
+    required this.messages,
+  });
+
+  factory BriefingConversation.fromJson(Map<String, dynamic> json) {
+    final rawMessages = json['messages'] as List<dynamic>? ?? [];
+    return BriefingConversation(
+      id: json['id'] as int,
+      title: json['title'] as String? ?? '',
+      briefingDate: json['briefing_date'] as String?,
+      messages: rawMessages
+          .map((e) => Message.fromJson(e as Map<String, dynamic>))
+          .toList(),
+    );
+  }
+
+  BriefingConversation copyWith({List<Message>? messages}) =>
+      BriefingConversation(
+        id: id,
+        title: title,
+        briefingDate: briefingDate,
+        messages: messages ?? this.messages,
+      );
+}