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.env.example

@@ -1,6 +1,9 @@
 # Required for SQL Server container and app DB connection
 SA_PASSWORD=ChangeThisToAStrongPassword123!
 
+# Public URL of the site (used to fix OAuth callbacks behind a reverse proxy)
+PUBLIC_URL=https://yourdomain.example.com
+
 # Optional OAuth (leave empty to disable provider at runtime)
 GOOGLE_CLIENT_ID=
 GOOGLE_CLIENT_SECRET=