debug(auth): log session cookie source

.env.example

@@ -15,6 +15,14 @@ WEB_PORT=3000
 # ======================
 NEXT_PUBLIC_APP_URL=http://localhost:3000
 NEXT_PUBLIC_API_URL=http://localhost:3001
+# Frontend auth mode:
+# - real: Normal auth/session flow
+# - mock: Local-only seeded user for FE development (blocked outside NODE_ENV=development)
+#   Use `mock` locally to continue FE work when auth flow is unstable.
+# If omitted, web runtime defaults:
+# - development -> mock
+# - production -> real
+NEXT_PUBLIC_AUTH_MODE=real
 
 # ======================
 # PostgreSQL Database