Add App__Domain config to derive connect redirect URI

Let deployments set a single App__Domain (e.g. sptb.example.com) instead of
spelling out the full ClientConnect__RedirectUri. The SharePoint-connect
callback is derived as <domain>/connect/callback; an explicit RedirectUri
still wins for back-compat.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -12,6 +12,10 @@ services:
     environment:
       - ASPNETCORE_ENVIRONMENT=Production
       - DataFolder=/data
+      # Public domain the app is reached at (e.g. sptb.example.com). The SharePoint-connect
+      # redirect URI (<domain>/connect/callback) is derived from it. Set your OIDC values
+      # here too, or pass an env file.
+      - App__Domain=${App__Domain:-}
     restart: unless-stopped
     healthcheck:
       # /account/login is anonymous and returns 200 (the app root now 302-redirects