fix default callback base URL port mismatch

Derive CALLBACK_BASE_URL fallback from LISTEN_ADDR so default local setup resolves to localhost:8080 and stays aligned with server bind port.

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

.env.example

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