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>

.env.example

@@ -6,6 +6,11 @@
 # Image tag to run (default: latest)
 SPTB_TAG=latest
 
+# Public domain the app is reached at (e.g. sptb.example.com or https://sptb.example.com).
+# Scheme defaults to https when omitted. The SharePoint-connect redirect URI is derived
+# from this as <domain>/connect/callback — register that on each client profile's app.
+# App__Domain=sptb.example.com
+
 # OIDC app sign-in (required in Production). Authority is derived from TenantId.
 Oidc__TenantId=00000000-0000-0000-0000-000000000000
 Oidc__ClientId=00000000-0000-0000-0000-000000000000