refactor: standardize project names to Musify.* and flatten layout

- Flatten nested per-repo folders into flat Musify.* projects at root
- Rename assemblies/csproj: Domain->Musify.Domain, Application->Musify.Application,
  Infrastructure->Musify.Infrastructure, WebApi->Musify.Api, Worker->Musify.Worker,
  StreamingGateway->Musify.StreamingGateway
- Rename WebApi.* namespaces to Musify.Api.*
- Update all ProjectReference paths and the solution file
- Consolidate stray per-repo dotfiles; add root .gitattributes
- Move flutter client out to _archive

Musify.Application/Users/Responses/UserApplicationResponse.cs

@@ -0,0 +1,26 @@
+using Musify.Domain.Entities;
+
+namespace Musify.Application.Users.Responses
+{
+    public record UserApplicationResponse(
+        long Id,
+        string Name,
+        string? FirstName,
+        string? SecondName,
+        string? ProfilePictureUrl,
+        DateTime CreatedAt,
+        DateTime UpdatedAt)
+    {
+        public static UserApplicationResponse FromEntity(User user)
+        {
+            return new UserApplicationResponse(
+                user.Id,
+                user.Name,
+                user.FirstName,
+                user.SecondName,
+                user.ProfilePictureUrl,
+                user.CreatedAt,
+                user.UpdatedAt);
+        }
+    }
+}