Add authentication for secure document access

Introduced a new login page (`Login.razor`) to handle user authentication via access codes. Implemented `AuthService` to validate access codes through an API. Updated `ReportViewer.razor` to check user access and redirect unauthorized users to the login page. Modified `Program.cs` to register `AuthService` for dependency injection. Enhanced document fetching in `ReportViewer.razor` to ensure secure access control.

EnvelopeGenerator.ReceiverUI/Services/AuthService.cs

@@ -0,0 +1,20 @@
+using System.Net;
+using EnvelopeGenerator.ReceiverUI.Options;
+using Microsoft.Extensions.Options;
+
+namespace EnvelopeGenerator.ReceiverUI.Services;
+
+public class AuthService(HttpClient http, IOptions<ApiOptions> apiOptions)
+{
+    private readonly ApiOptions _api = apiOptions.Value;
+
+    /// <summary>
+    /// Checks whether the current user holds a valid receiver token for the given envelope key.
+    /// Calls GET /api/auth/check/envelope/{envelopeKey}.
+    /// </summary>
+    public async Task<bool> CheckEnvelopeAccessAsync(string envelopeKey, CancellationToken cancel = default)
+    {
+        var response = await http.GetAsync($"{_api.BaseUrl}/api/auth/check/envelope/{Uri.EscapeDataString(envelopeKey)}", cancel);
+        return response.StatusCode == HttpStatusCode.OK;
+    }
+}