fix: resolve all flutter analyze warnings and infos

- app.dart: add braces to single-statement if body
- briefing_api.dart: escape <id> in doc comment (unintended HTML)
- notes_api.dart, tasks_api.dart, projects_api.dart: use null-aware
  map elements (?'key': value) instead of if-null guards
- task_edit_screen.dart: remove unused tasks_api.dart import
- task_edit_screen.dart, project_selector.dart: suppress
  deprecated_member_use on DropdownButtonFormField.value (value is
  the controlled-widget param; switching to initialValue would break
  current-selection display)
- project_selector.dart: use _ instead of __ for ignored error params

lib/data/api/briefing_api.dart

@@ -35,7 +35,7 @@ class BriefingApi {
     }
   }
 
-  /// GET /api/briefing/conversations/<id>/messages
+  /// GET /api/briefing/conversations/`<id>`/messages
   Future<List<Message>> getMessages(int convId) async {
     try {
       final response =