Add signed result page and update navigation flow

Introduced a new `/envelope/signed-result` page to confirm
successful document signing with localized messages and
a visually appealing UI. Updated `ReceiverSignedPage.razor`
to redirect users to the new page after signing.

Enhanced styles in `app.css` for the signed result page,
including responsive layout, theme-specific designs, and
typography improvements. Added support for localization
using `IStringLocalizer` with fallback strings in German.

Improves user experience by providing clear feedback and
a polished confirmation page after signing.

EnvelopeGenerator.Server/EnvelopeGenerator.Server/Components/Pages/ReceiverSignedPage.razor

@@ -174,7 +174,7 @@
         _submitConfirmVisible = false;
         await InvokeAsync(StateHasChanged);
         await AuthService.LogoutEnvelopeReceiverAsync(EnvelopeKey!);
-        Navigation.NavigateTo("/", forceLoad: true);
+        Navigation.NavigateTo("/envelope/signed-result", forceLoad: true);
     }
 
     protected override async Task OnInitializedAsync()