Add feature to open receiver envelope URLs

Introduce a new feature allowing users to open URLs for receiver envelopes directly from the sender page. This includes:

- Injecting `EnvelopeReceiverService` in `SenderPage.razor`.
- Adding a UI button to open receiver envelope URLs.
- Implementing `OpenReceiverEnvelopeUrlAsync` for URL handling.
- Extending `EnvelopeReceiverService` with methods for URL retrieval and feature check.
- Updating `EnvelopeReceiverController` with new endpoints for URL access.
- Introducing `SenderFeatureAccessOptions` for managing feature access.
- Configuring feature access in `Program.cs` and `appsettings.json`.
- Restricting feature usage to specified email addresses.

EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Services/EnvelopeReceiverService.cs

@@ -15,6 +15,8 @@ namespace EnvelopeGenerator.Server.Client.Services;
 public class EnvelopeReceiverService(IHttpClientFactory httpClientFactory)
 {
     private static readonly JsonSerializerOptions _jsonOptions = new(JsonSerializerDefaults.Web);
+    private sealed record ReceiverEnvelopeUrlResponse(string Url);
+    private sealed record ReceiverEnvelopeUrlEnabledResponse(bool Enabled);
 
     /// <summary>
     /// Fetches the envelope receiver data for the given envelope key from the API.
@@ -63,4 +65,32 @@ public class EnvelopeReceiverService(IHttpClientFactory httpClientFactory)
 
         return await response.Content.ReadFromJsonAsync<CreateEnvelopeReceiverResponse>(_jsonOptions, cancel);
     }
+
+    public async Task<string?> GetReceiverEnvelopeUrlAsync(string envelopeUuid, string receiverSignature, CancellationToken cancel = default)
+    {
+        if (string.IsNullOrWhiteSpace(envelopeUuid) || string.IsNullOrWhiteSpace(receiverSignature))
+            return null;
+
+        using var http = httpClientFactory.CreateClient("EnvelopeGenerator.Server");
+        var url = $"/api/EnvelopeReceiver/receiver-envelope-url?envelopeUuid={Uri.EscapeDataString(envelopeUuid)}&receiverSignature={Uri.EscapeDataString(receiverSignature)}";
+        var response = await http.GetAsync(url, cancel);
+
+        if (!response.IsSuccessStatusCode)
+            return null;
+
+        var payload = await response.Content.ReadFromJsonAsync<ReceiverEnvelopeUrlResponse>(_jsonOptions, cancel);
+        return payload?.Url;
+    }
+
+    public async Task<bool> GetReceiverEnvelopeUrlFeatureEnabledAsync(CancellationToken cancel = default)
+    {
+        using var http = httpClientFactory.CreateClient("EnvelopeGenerator.Server");
+        var response = await http.GetAsync("/api/EnvelopeReceiver/receiver-envelope-url/enabled", cancel);
+
+        if (!response.IsSuccessStatusCode)
+            return false;
+
+        var payload = await response.Content.ReadFromJsonAsync<ReceiverEnvelopeUrlEnabledResponse>(_jsonOptions, cancel);
+        return payload?.Enabled == true;
+    }
 }