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/Controllers/UserPreferencesController.cs

@@ -0,0 +1,49 @@
+using Microsoft.AspNetCore.Authorization;
+using Microsoft.AspNetCore.Mvc;
+using EnvelopeGenerator.Domain.Constants;
+using EnvelopeGenerator.Server.Extensions;
+using EnvelopeGenerator.Application.Common.Dto;
+using EnvelopeGenerator.Application.Common.Interfaces.Services;
+
+namespace EnvelopeGenerator.Server.Controllers;
+
+/// <summary>
+/// Manages per-user UI preferences (theme, dark mode, grid layouts).
+/// Preferences are stored in the distributed cache (dbo.TBDD_CACHE).
+/// </summary>
+[ApiController]
+[Route("api/[controller]")]
+[Authorize(Policy = AuthPolicy.Sender)]
+public class UserPreferencesController(IUserPreferencesService preferencesService) : ControllerBase
+{
+    private static readonly HashSet<string> AllowedThemes =
+        ["blazing-berry", "fluent", "purple"];
+
+    /// <summary>
+    /// Returns the current user's UI preferences.
+    /// Always returns 200 with defaults if no preferences have been saved yet.
+    /// </summary>
+    [HttpGet]
+    public async Task<ActionResult<UserPreferencesDto>> Get(CancellationToken ct)
+    {
+        var username = User.GetUsername();
+        var prefs = await preferencesService.GetAsync(username, ct);
+        return Ok(prefs);
+    }
+
+    /// <summary>
+    /// Saves (full replace) the current user's UI preferences.
+    /// </summary>
+    [HttpPut]
+    public async Task<ActionResult<UserPreferencesDto>> Put(
+        [FromBody] UserPreferencesDto dto,
+        CancellationToken ct)
+    {
+        if (!AllowedThemes.Contains(dto.ThemeName))
+            return BadRequest($"Invalid theme '{dto.ThemeName}'. Allowed: {string.Join(", ", AllowedThemes)}.");
+
+        var username = User.GetUsername();
+        await preferencesService.SaveAsync(username, dto, ct);
+        return Ok(dto);
+    }
+}

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>();