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.Application/Common/Interfaces/Services/IUserPreferencesService.cs

@@ -0,0 +1,29 @@
+using EnvelopeGenerator.Application.Common.Dto;
+
+namespace EnvelopeGenerator.Application.Common.Interfaces.Services;
+
+/// <summary>
+/// Manages per-user UI preferences (theme, dark mode, grid layouts).
+/// Preferences are stored in the distributed cache (dbo.TBDD_CACHE) as JSON.
+/// Cache key schema: "signflow.user-preferences:{username}"
+/// </summary>
+public interface IUserPreferencesService
+{
+    /// <summary>
+    /// Retrieves preferences for the given user.
+    /// Always returns a valid object — never throws, never returns null.
+    /// Returns defaults if no entry exists in cache.
+    /// </summary>
+    /// <param name="username">The sender's username (ClaimTypes.Name from JWT).</param>
+    /// <param name="ct">Cancellation token.</param>
+    Task<UserPreferencesDto> GetAsync(string username, CancellationToken ct = default);
+
+    /// <summary>
+    /// Persists the given preferences for the user in the distributed cache.
+    /// Failures are non-fatal and silently ignored.
+    /// </summary>
+    /// <param name="username">The sender's username (ClaimTypes.Name from JWT).</param>
+    /// <param name="preferences">The preferences to persist.</param>
+    /// <param name="ct">Cancellation token.</param>
+    Task SaveAsync(string username, UserPreferencesDto preferences, CancellationToken ct = default);
+}