feat(account): add /account/update_name endpoint

Implements POST /account/update_name — writes Viewer.DisplayName and
returns an empty array per the prod capture. Includes TDD test covering
the persist side-effect.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

SVSim.EmulatedEntrypoint/Controllers/AccountController.cs

@@ -0,0 +1,33 @@
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.EntityFrameworkCore;
+using SVSim.Database;
+using SVSim.EmulatedEntrypoint.Models.Dtos.Requests.Account;
+
+namespace SVSim.EmulatedEntrypoint.Controllers;
+
+/// <summary>
+/// /account/* — viewer profile mutations that aren't tied to a specific subsystem.
+/// </summary>
+public class AccountController : SVSimController
+{
+    private readonly SVSimDbContext _db;
+
+    public AccountController(SVSimDbContext db)
+    {
+        _db = db;
+    }
+
+    [HttpPost("update_name")]
+    public async Task<IActionResult> UpdateName([FromBody] AccountUpdateNameRequest request)
+    {
+        if (!TryGetViewerId(out long viewerId)) return Unauthorized();
+
+        var viewer = await _db.Viewers.FirstAsync(v => v.Id == viewerId);
+        viewer.DisplayName = request.Name;
+        await _db.SaveChangesAsync();
+
+        // Prod returns `data: []` — empty array, not empty object. Use an empty array literal
+        // so the translation middleware emits the right msgpack shape.
+        return Ok(Array.Empty<object>());
+    }
+}

SVSim.EmulatedEntrypoint/Models/Dtos/Requests/Account/AccountUpdateNameRequest.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 AccountUpdateNameRequest : BaseRequest
+{
+    [JsonPropertyName("name")]
+    [Key("name")]
+    public string Name { get; set; } = string.Empty;
+}