Refactor envelope receiver auth flow and state handling

- Introduce IReceiverAuthService and ReceiverAuthService for all envelope receiver authentication and status API calls
- Add ReceiverAuthModel as a client-side DTO for API responses
- Refactor EnvelopeState to store all relevant fields and update via ApplyApiResponse
- Overhaul EnvelopePage.razor to use new service and state, with improved status handling and UI
- Enhance ApiResponse and ApiServiceBase to support structured error deserialization
- Register IReceiverAuthService in DI container

EnvelopeGenerator.ReceiverUI/EnvelopeGenerator.ReceiverUI.Client/Models/ReceiverAuthModel.cs

@@ -0,0 +1,31 @@
+namespace EnvelopeGenerator.ReceiverUI.Client.Models;
+
+/// <summary>
+/// Client-seitiges DTO für die Antwort des ReceiverAuthControllers.
+/// Wird 1:1 aus dem JSON deserialisiert.
+/// 
+/// WARUM ein eigenes Client-Model statt das API-Model zu referenzieren?
+/// - Das API-Projekt hat Server-Abhängigkeiten (EF Core, SqlClient, etc.)
+/// - Diese Pakete existieren nicht für browser-wasm → Build-Fehler
+/// - Die Property-Namen müssen nur zum JSON passen (case-insensitive)
+/// </summary>
+public record ReceiverAuthModel
+{
+    /// <summary>
+    /// Aktueller Status des Empfänger-Flows.
+    /// Werte: "requires_access_code", "requires_tfa", "show_document",
+    ///        "already_signed", "rejected", "not_found", "expired", "error"
+    /// </summary>
+    public string Status { get; init; } = string.Empty;
+
+    public string? Title { get; init; }
+    public string? Message { get; init; }
+    public string? SenderEmail { get; init; }
+    public string? ReceiverName { get; init; }
+    public bool TfaEnabled { get; init; }
+    public bool HasPhoneNumber { get; init; }
+    public bool ReadOnly { get; init; }
+    public string? TfaType { get; init; }
+    public DateTime? TfaExpiration { get; init; }
+    public string? ErrorMessage { get; init; }
+}