feat(account): /account/update_birth persists viewer birth date

Strict yyyy-MM-dd parse (matches /load/index round-trip format) — rejects
malformed input. data_headers.servertime is emitted by the standard
envelope, which is what the client reads into BirthDayUpdateServerTime.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

SVSim.EmulatedEntrypoint/Models/Dtos/Requests/Account/AccountUpdateBirthRequest.cs

@@ -0,0 +1,13 @@
+using System.Text.Json.Serialization;
+using MessagePack;
+using SVSim.EmulatedEntrypoint.Models.Dtos.Requests;
+
+namespace SVSim.EmulatedEntrypoint.Models.Dtos.Requests.Account;
+
+[MessagePackObject]
+public class AccountUpdateBirthRequest : BaseRequest
+{
+    [JsonPropertyName("birth")]
+    [Key("birth")]
+    public string Birth { get; set; } = string.Empty;
+}