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/briefing_provider.dart

@@ -0,0 +1,113 @@
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+import '../data/models/briefing_conversation.dart';
+import '../data/models/message.dart';
+import 'api_client_provider.dart';
+
+/// Drives the loading indicator in BriefingScreen's reply area.
+final isBriefingStreamingProvider = StateProvider<bool>((ref) => false);
+
+final briefingProvider =
+    AsyncNotifierProvider<BriefingNotifier, BriefingConversation>(
+        BriefingNotifier.new);
+
+class BriefingNotifier extends AsyncNotifier<BriefingConversation> {
+  @override
+  Future<BriefingConversation> build() async {
+    return ref.read(briefingApiProvider).getToday();
+  }
+
+  /// Trigger a briefing slot (e.g. "compilation") then reload.
+  Future<void> refresh(String slot) async {
+    await ref.read(briefingApiProvider).triggerSlot(slot);
+    ref.invalidateSelf();
+    await future;
+  }
+
+  /// Send a reply to today's briefing conversation.
+  ///
+  /// Mirrors MessagesNotifier.sendMessage():
+  ///   1. Optimistic UI update
+  ///   2. POST message to chat endpoint
+  ///   3. SSE stream (best-effort)
+  ///   4. Poll until complete
+  Future<void> sendReply(String content) async {
+    final conv = state.valueOrNull;
+    if (conv == null) return;
+    final convId = conv.id;
+    final chatApi = ref.read(chatApiProvider);
+
+    final previous = conv.messages;
+    final userMsg = Message(
+      conversationId: convId,
+      role: MessageRole.user,
+      content: content,
+    );
+    final placeholder = Message(
+      conversationId: convId,
+      role: MessageRole.assistant,
+      content: '',
+      status: 'generating',
+    );
+    state = AsyncData(conv.copyWith(messages: [...previous, userMsg, placeholder]));
+    ref.read(isBriefingStreamingProvider.notifier).state = true;
+
+    try {
+      await chatApi.sendMessage(convId, content);
+    } catch (e) {
+      state = AsyncData(conv.copyWith(messages: previous));
+      ref.read(isBriefingStreamingProvider.notifier).state = false;
+      rethrow;
+    }
+
+    // SSE stream (best-effort)
+    bool streamedContent = false;
+    try {
+      await for (final chunk in chatApi.streamGeneration(convId)) {
+        streamedContent = true;
+        final current = state.valueOrNull;
+        if (current == null) break;
+        final msgs = current.messages;
+        if (msgs.isEmpty) continue;
+        final updated = msgs.last.copyWith(content: msgs.last.content + chunk);
+        state = AsyncData(current.copyWith(
+            messages: [...msgs.sublist(0, msgs.length - 1), updated]));
+      }
+    } catch (_) {
+      // Fall through to polling.
+    }
+
+    // Poll until complete (max 20 attempts, 2s apart)
+    try {
+      for (var attempt = 0; attempt < 20; attempt++) {
+        if (attempt > 0) await Future.delayed(const Duration(seconds: 2));
+        final fresh = await ref.read(briefingApiProvider).getMessages(convId);
+        final done = fresh.any(
+          (m) => m.role == MessageRole.assistant && m.status != 'generating',
+        );
+        final hasContent = fresh.any(
+          (m) => m.role == MessageRole.assistant && m.content.isNotEmpty,
+        );
+        final current = state.valueOrNull;
+        if (current != null && (!streamedContent || done || hasContent)) {
+          state = AsyncData(current.copyWith(messages: fresh));
+        }
+        if (done) break;
+      }
+    } catch (_) {
+      // Clear the generating placeholder so UI doesn't spin forever.
+      final current = state.valueOrNull;
+      if (current != null) {
+        final msgs = current.messages;
+        if (msgs.isNotEmpty && msgs.last.status == 'generating') {
+          state = AsyncData(current.copyWith(messages: [
+            ...msgs.sublist(0, msgs.length - 1),
+            msgs.last.copyWith(status: 'complete'),
+          ]));
+        }
+      }
+    } finally {
+      ref.read(isBriefingStreamingProvider.notifier).state = false;
+    }
+  }
+}