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>

appsettings.json

@@ -1,5 +1,8 @@
 {
   "DataFolder": "/data",
+  "App": {
+    "Domain": ""
+  },
   "Oidc": {
     "TenantId": "YOUR_ENTRA_TENANT_ID",
     "ClientId": "YOUR_SPTB_APP_CLIENT_ID",