Add reusable UI components and toast notification system

- Introduce ActionPanel, EnvelopeInfoCard, TfaForm, ConfirmDialog, StatusPage, and Toast components for modular, presentational UI
- Add ToastService for pub/sub toast notifications; register in DI
- Refactor AccessCodeForm for improved UX and parameterization
- Enhance MainLayout with Toast integration and better error handling
- Standardize and extend app.css for new components and responsive design
- All new components are "dumb" (no service/API knowledge), using EventCallbacks for parent interaction
- ConfirmDialog supports awaitable user confirmation via TaskCompletionSource

EnvelopeGenerator.ReceiverUI/EnvelopeGenerator.ReceiverUI.Client/Program.cs

@@ -24,4 +24,6 @@ builder.Services.AddScoped<IEnvelopeService, EnvelopeService>();
 // State: Ein State-Objekt pro Browser-Tab
 builder.Services.AddScoped<EnvelopeState>();
 
+builder.Services.AddScoped<ToastService>();
+
 await builder.Build().RunAsync();