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/projects_api.dart

@@ -44,7 +44,7 @@ class ProjectsApi {
         if (description != null && description.isNotEmpty)
           'description': description,
         if (goal != null && goal.isNotEmpty) 'goal': goal,
-        if (color != null) 'color': color,
+        ?'color': color,
       });
       return Project.fromJson(response.data as Map<String, dynamic>);
     } on DioException catch (e) {