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 =

lib/data/api/notes_api.dart

@@ -38,7 +38,7 @@ class NotesApi {
         'title': title,
         'body': body,
         'tags': tags,
-        if (projectId != null) 'project_id': projectId,
+        ?'project_id': projectId,
       });
       return Note.fromJson(response.data as Map<String, dynamic>);
     } on DioException catch (e) {
@@ -59,7 +59,7 @@ class NotesApi {
         'title': title,
         'body': body,
         'tags': tags,
-        if (clearProject) 'project_id': null else if (projectId != null) 'project_id': projectId,
+        if (clearProject) 'project_id': null else ?'project_id': projectId,
       });
       return Note.fromJson(response.data as Map<String, dynamic>);
     } on DioException catch (e) {

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) {

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) {