Add user preferences management via distributed cache

Introduced `UserPreferencesDto` to represent UI preferences,
including theme, dark mode, and grid layouts. Added the
`IUserPreferencesService` interface and implemented it in
`UserPreferencesService` to manage preferences stored in
the distributed cache (`dbo.TBDD_CACHE`).

Registered `UserPreferencesService` in dependency injection
and added a NuGet reference for `Microsoft.Extensions.Caching.Abstractions`.

Created `UserPreferencesController` with `GET` and `PUT`
endpoints to retrieve and save user preferences. Validated
theme names in the `PUT` endpoint.

Updated `Program.cs` to ensure proper service registration.

EnvelopeGenerator.Server/EnvelopeGenerator.Server/Program.cs

@@ -318,6 +318,9 @@ try
         .AddEnvelopeGeneratorServices(config);
 #pragma warning restore CS0618
 
+    // User Preferences (theme, dark mode, grid layouts — stored in TBDD_CACHE)
+    // Registration handled by Infrastructure DependencyInjection (AddEnvelopeGeneratorInfrastructureServices)
+
     // Business Services (Server specific)
     builder.Services.AddScoped<DocumentService>();
     builder.Services.AddScoped<AuthService>();