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/screens/chat/chat_screen.dart

@@ -1,12 +1,9 @@
-import 'dart:math' show min;
-
 import 'package:flutter/material.dart';
-import 'package:flutter_markdown_plus/flutter_markdown_plus.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
 import '../../core/exceptions.dart';
-import '../../data/models/message.dart';
 import '../../providers/chat_provider.dart';
+import '../../widgets/chat_message_bubble.dart';
 
 class ChatScreen extends ConsumerStatefulWidget {
   final int conversationId;
@@ -102,7 +99,7 @@ class _ChatScreenState extends ConsumerState<ChatScreen> {
                       horizontal: 8, vertical: 12),
                   itemCount: messages.length,
                   itemBuilder: (context, i) =>
-                      _MessageBubble(message: messages[i]),
+                      ChatMessageBubble(message: messages[i]),
                 );
               },
             ),
@@ -153,42 +150,3 @@ class _ChatScreenState extends ConsumerState<ChatScreen> {
   }
 }
 
-class _MessageBubble extends StatelessWidget {
-  final Message message;
-  const _MessageBubble({required this.message});
-
-  @override
-  Widget build(BuildContext context) {
-    final isUser = message.role == MessageRole.user;
-    final scheme = Theme.of(context).colorScheme;
-
-    return Align(
-      alignment: isUser ? Alignment.centerRight : Alignment.centerLeft,
-      child: Container(
-        constraints: BoxConstraints(
-          maxWidth: min(MediaQuery.of(context).size.width * 0.8, 480),
-        ),
-        margin: const EdgeInsets.symmetric(vertical: 4),
-        padding: const EdgeInsets.symmetric(horizontal: 12, vertical: 8),
-        decoration: BoxDecoration(
-          color: isUser ? scheme.primary : scheme.surfaceContainerHighest,
-          borderRadius: BorderRadius.only(
-            topLeft: const Radius.circular(16),
-            topRight: const Radius.circular(16),
-            bottomLeft: Radius.circular(isUser ? 16 : 4),
-            bottomRight: Radius.circular(isUser ? 4 : 16),
-          ),
-        ),
-        child: isUser
-            ? Text(
-                message.content,
-                style: TextStyle(color: scheme.onPrimary),
-              )
-            : MarkdownBody(
-                data: message.content.isEmpty ? '...' : message.content,
-                styleSheet: MarkdownStyleSheet.fromTheme(Theme.of(context)),
-              ),
-      ),
-    );
-  }
-}