Add search, offline queue, app icon, and UI polish

- Collapsible search in notes and tasks: magnifying glass in AppBar
  expands to a text field inline; close button resets the filter
- Offline capture queue: failed quick-captures (NetworkException) are
  persisted to SharedPreferences and retried automatically on next
  successful submit or app start; badge shows pending count
- App icon: book-with-sparkle logo from FabledAssistant SVG rendered
  at all Android densities with adaptive icon (indigo #6366f1 bg)
- Dark mode subtitle fix: use colorScheme.onSurfaceVariant for note
  preview and conversation timestamp text
- Remove swipe-to-delete (accidental deletions); long-press remains
- Chat: SSE streaming reliability, polling fallback, title patching
- Settings: theme toggle (system/light/dark) persisted to prefs
- Notes: delete button in edit screen; body preview in list
- Tasks: fix delete dialog context; description search support

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

lib/data/models/conversation.dart

@@ -13,7 +13,7 @@ class Conversation {
 
   factory Conversation.fromJson(Map<String, dynamic> json) => Conversation(
         id: json['id'] as int,
-        title: json['title'] as String,
+        title: json['title'] as String? ?? '',
         createdAt: DateTime.parse(json['created_at'] as String),
         updatedAt: DateTime.parse(json['updated_at'] as String),
       );