Introduced `EnvelopeAuthService` and `IEnvelopeAuthService` to handle server-side authentication for envelope receiver pages.
- Registered `IEnvelopeAuthService` as a scoped service in `Program.cs`.
- Implemented `EnvelopeAuthService` to validate user authentication and envelope key matching using `IHttpContextAccessor` and JWT claims.
- Added methods to retrieve the authenticated envelope key and current user (`ClaimsPrincipal`).
- Prioritized `NameIdentifier` claim for envelope key extraction, with fallback to `sub` claim.
- Documented the service and interface with XML comments for clarity.
This centralizes authentication logic, ensuring reusability and adherence to SSR best practices.
TekH
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