Add PublicUrl support for reverse proxy handling

Introduced a `PUBLIC_URL` environment variable to configure the
public URL for OAuth callbacks and reverse proxy scenarios. Updated
`Program.cs` to rewrite request scheme and host based on `PublicUrl`.
Replaced `Microsoft.AspNetCore.Components.Authorization` with
`Microsoft.AspNetCore.Components.Authentication`. Added `PublicUrl`
to `appsettings.json` with a default empty value.

deploy/portainer-stack.yml

@@ -22,6 +22,7 @@ services:
       - Smtp__Password=${SMTP_PASSWORD}
       - Smtp__FromAddress=${SMTP_FROM_ADDRESS}
       - Smtp__FromName=${SMTP_FROM_NAME}
+      - PublicUrl=${PUBLIC_URL}
     depends_on:
       - mssql
     networks: