Move form state ownership from Home to leaf controls

RpgRoller/Components/Pages/Home.Models.cs

@@ -14,6 +14,13 @@ public sealed class FormState<TModel>
     }
 }
 
+public sealed class FormSubmissionResult
+{
+    public Dictionary<string, string> Errors { get; init; } = [];
+    public string? ErrorMessage { get; init; }
+    public bool IsSuccess => Errors.Count == 0 && string.IsNullOrWhiteSpace(ErrorMessage);
+}
+
 public sealed class RegisterFormModel
 {
     public string Username { get; set; } = string.Empty;