fix callback URL to use configurable public base

Separate callback URL generation from LISTEN_ADDR by introducing CALLBACK_BASE_URL with a localhost default for local development.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.env.example

@@ -22,3 +22,4 @@ WHITELIST_EMAILS=user1@example.com,user2@example.com
 # === Optional ===
 IMAP_PROXY_URL=
 LISTEN_ADDR=:8080
+CALLBACK_BASE_URL=http://localhost