Refactor AuthService and add new service classes

Refactored `AuthService` to introduce a reusable `CreateDefaultClient` method, reducing code duplication. Updated all relevant methods in `AuthService` to use this new method.

Added `CultureService` to manage application culture/localization, including support for setting, getting, and initializing culture from `localStorage` or browser settings.

Introduced `DocReceiverElementService` for retrieving document receiver elements (signatures) and `EnvelopeService` for managing envelope data retrieval with optional filters. Both services include error handling and consistent JSON deserialization.

These changes improve code maintainability, reusability, and adhere to the single responsibility principle.

EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Services/EnvelopeService.cs

@@ -0,0 +1,72 @@
+using System.Net.Http.Json;
+using System.Text.Json;
+using EnvelopeGenerator.Server.Client.Models;
+using EnvelopeGenerator.Server.Client.Options;
+using Microsoft.AspNetCore.WebUtilities;
+using Microsoft.Extensions.Options;
+
+namespace EnvelopeGenerator.Server.Client.Services;
+
+/// <summary>
+/// Retrieves <see cref="EnvelopeDto"/>s from the API.
+/// </summary>
+public class EnvelopeService
+{
+    private readonly HttpClient _http;
+    private readonly ApiOptions _apiOptions;
+    private static readonly JsonSerializerOptions _jsonOptions = new(JsonSerializerDefaults.Web);
+
+    public EnvelopeService(HttpClient http, IOptions<ApiOptions> apiOptions)
+    {
+        _http = http;
+        _apiOptions = apiOptions.Value;
+    }
+
+    /// <summary>
+    /// Fetches envelopes from the API with optional filters.
+    /// </summary>
+    /// <exception cref="HttpRequestException">Thrown when the API request fails.</exception>
+    public async Task<IEnumerable<EnvelopeDto>?> GetAsync(
+        int? id = null,
+        string? uuid = null,
+        bool? onlyActive = null,
+        bool? onlyCompleted = null,
+        CancellationToken cancel = default)
+    {
+        var baseUrl = $"{_apiOptions.BaseUrl}/api/Envelope";
+        var queryParams = new Dictionary<string, string?>();
+
+        if (id.HasValue)
+        {
+            queryParams["Id"] = id.Value.ToString();
+        }
+        if (!string.IsNullOrEmpty(uuid))
+        {
+            queryParams["Uuid"] = uuid;
+        }
+        if (onlyActive.HasValue)
+        {
+            queryParams["OnlyActive"] = onlyActive.Value.ToString();
+        }
+        if (onlyCompleted.HasValue)
+        {
+            queryParams["OnlyCompleted"] = onlyCompleted.Value.ToString();
+        }
+
+        var url = QueryHelpers.AddQueryString(baseUrl, queryParams);
+
+        var response = await _http.GetAsync(url, cancel);
+
+        if (!response.IsSuccessStatusCode)
+        {
+            var statusCode = (int)response.StatusCode;
+            var reasonPhrase = response.ReasonPhrase ?? "Unknown error";
+            throw new HttpRequestException(
+                $"Failed to load envelopes. Status: {statusCode} ({reasonPhrase})",
+                null,
+                response.StatusCode);
+        }
+
+        return await response.Content.ReadFromJsonAsync<IEnumerable<EnvelopeDto>>(_jsonOptions, cancel);
+    }
+}