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/tasks_api.dart

@@ -43,8 +43,8 @@ class TasksApi {
         'status': status.value,
         'priority': priority.value,
         'due_date': dueDate?.toIso8601String(),
-        if (projectId != null) 'project_id': projectId,
-        if (parentId != null) 'parent_id': parentId,
+        ?'project_id': projectId,
+        ?'parent_id': parentId,
       });
       return Task.fromJson(response.data as Map<String, dynamic>);
     } on DioException catch (e) {