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/widgets/briefing_digest_card.dart

@@ -0,0 +1,133 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_markdown_plus/flutter_markdown_plus.dart';
+
+import '../data/models/message.dart';
+
+class BriefingDigestCard extends StatefulWidget {
+  /// The first assistant message from today's briefing, or null if none yet.
+  final Message? message;
+
+  /// Called when the user taps "Generate now".
+  final VoidCallback? onGenerateNow;
+
+  const BriefingDigestCard({
+    super.key,
+    required this.message,
+    this.onGenerateNow,
+  });
+
+  @override
+  State<BriefingDigestCard> createState() => _BriefingDigestCardState();
+}
+
+class _BriefingDigestCardState extends State<BriefingDigestCard> {
+  bool _expanded = false;
+
+  @override
+  Widget build(BuildContext context) {
+    final scheme = Theme.of(context).colorScheme;
+    final textTheme = Theme.of(context).textTheme;
+
+    return Card(
+      margin: const EdgeInsets.fromLTRB(12, 8, 12, 4),
+      elevation: 0,
+      shape: RoundedRectangleBorder(
+        borderRadius: BorderRadius.circular(14),
+        side: BorderSide(
+          color: scheme.outlineVariant.withValues(alpha: 0.5),
+          width: 1,
+        ),
+      ),
+      child: Padding(
+        padding: const EdgeInsets.all(16),
+        child: Column(
+          crossAxisAlignment: CrossAxisAlignment.start,
+          children: [
+            // Header row
+            Row(
+              children: [
+                Icon(Icons.wb_sunny_outlined, size: 18, color: scheme.primary),
+                const SizedBox(width: 8),
+                Text(
+                  _todayLabel(),
+                  style: textTheme.labelMedium?.copyWith(
+                    color: scheme.onSurfaceVariant,
+                  ),
+                ),
+              ],
+            ),
+            const SizedBox(height: 10),
+
+            // Body
+            if (widget.message == null) ...[
+              Text(
+                'No briefing yet today.',
+                style: textTheme.bodyMedium?.copyWith(
+                  color: scheme.onSurfaceVariant,
+                ),
+              ),
+              const SizedBox(height: 12),
+              if (widget.onGenerateNow != null)
+                FilledButton.tonal(
+                  onPressed: widget.onGenerateNow,
+                  child: const Text('Generate now'),
+                ),
+            ] else ...[
+              AnimatedSize(
+                duration: const Duration(milliseconds: 250),
+                curve: Curves.easeInOut,
+                alignment: Alignment.topCenter,
+                child: _expanded
+                    ? MarkdownBody(data: widget.message!.content)
+                    : _TruncatedMarkdown(
+                        data: widget.message!.content,
+                        maxLines: 5,
+                      ),
+              ),
+              const SizedBox(height: 8),
+              GestureDetector(
+                onTap: () => setState(() => _expanded = !_expanded),
+                child: Text(
+                  _expanded ? 'Show less ↑' : 'Show more ↓',
+                  style: TextStyle(
+                    color: scheme.primary,
+                    fontSize: 13,
+                    fontWeight: FontWeight.w500,
+                  ),
+                ),
+              ),
+            ],
+          ],
+        ),
+      ),
+    );
+  }
+
+  String _todayLabel() {
+    final now = DateTime.now();
+    const days = [
+      'Monday', 'Tuesday', 'Wednesday', 'Thursday',
+      'Friday', 'Saturday', 'Sunday'
+    ];
+    const months = [
+      'January', 'February', 'March', 'April', 'May', 'June',
+      'July', 'August', 'September', 'October', 'November', 'December'
+    ];
+    return '${days[now.weekday - 1]}, ${months[now.month - 1]} ${now.day}';
+  }
+}
+
+/// Renders Markdown truncated to [maxLines] visible lines.
+class _TruncatedMarkdown extends StatelessWidget {
+  final String data;
+  final int maxLines;
+  const _TruncatedMarkdown({required this.data, required this.maxLines});
+
+  @override
+  Widget build(BuildContext context) {
+    return ConstrainedBox(
+      constraints: BoxConstraints(maxHeight: maxLines * 20.0),
+      child: ClipRect(child: MarkdownBody(data: data)),
+    );
+  }
+}