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.Infrastructure/UserPreferencesService.cs

@@ -0,0 +1,41 @@
+#if NET
+using System.Text.Json;
+using Microsoft.Extensions.Caching.Distributed;
+using EnvelopeGenerator.Application.Common.Dto;
+using EnvelopeGenerator.Application.Common.Interfaces.Services;
+
+namespace EnvelopeGenerator.Infrastructure;
+
+/// <summary>
+/// Implements user preferences storage using the distributed cache (dbo.TBDD_CACHE).
+/// No DB schema changes required — uses the existing SQL Server distributed cache table.
+/// Cache key schema: "signflow.user-preferences:{username}"
+/// Expiration: indefinite (no TTL) — preferences persist until explicitly overwritten.
+/// </summary>
+public sealed class UserPreferencesService(IDistributedCache cache) : IUserPreferencesService
+{
+    private const string KeyPrefix = "signflow.user-preferences:";
+
+    private static string BuildKey(string username) => $"{KeyPrefix}{username}";
+
+    /// <inheritdoc />
+    public async Task<UserPreferencesDto> GetAsync(string username, CancellationToken ct = default)
+    {
+        var json = await cache.GetStringAsync(BuildKey(username), ct);
+
+        if (json is null)
+            return new UserPreferencesDto();
+
+        return JsonSerializer.Deserialize<UserPreferencesDto>(json) ?? new UserPreferencesDto();
+    }
+
+    /// <inheritdoc />
+    public async Task SaveAsync(string username, UserPreferencesDto preferences, CancellationToken ct = default)
+    {
+        var json = JsonSerializer.Serialize(preferences);
+
+        // No expiration — preferences are stored indefinitely.
+        await cache.SetStringAsync(BuildKey(username), json, new DistributedCacheEntryOptions(), ct);
+    }
+}
+#endif