Added an authentication check in `EnvelopeSenderPage.razor` to verify sender access before loading envelopes. Redirects unauthorized users to the sender login page.
Introduced `CheckSenderAsync` in `AuthService` to validate sender tokens via the `/api/auth/check` endpoint. Updated `OnInitializedAsync` to use this method, enhancing security by ensuring only authorized users can access envelope-related functionality.
TekH
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