Add EnvelopeAuthService for SSR authentication

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.

EnvelopeGenerator.Server/EnvelopeGenerator.Server/Program.cs

@@ -319,6 +319,9 @@ try
     builder.Services.AddScoped<SignatureCacheService>();
     builder.Services.AddSingleton<AppVersionService>();
 
+    // SSR Authentication Service (for Envelope Receiver pages)
+    builder.Services.AddScoped<EnvelopeGenerator.Server.Services.IEnvelopeAuthService, EnvelopeGenerator.Server.Services.EnvelopeAuthService>();
+
     // DevExpress Server-Side Services (CRITICAL for DxPdfViewer)
     builder.Services.AddDevExpressBlazor();
     builder.Services.AddDevExpressServerSideBlazorPdfViewer();