Fix frontend healthcheck: use 127.0.0.1 instead of localhost

Alpine resolves localhost to IPv6 [::1] but nginx only listens on
IPv4, causing the healthcheck to fail with connection refused.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker-compose.yaml

@@ -62,7 +62,7 @@ services:
     networks:
       - frontend_net
     healthcheck:
-      test: ["CMD", "wget", "--spider", "--quiet", "http://localhost:8080/"]
+      test: ["CMD", "wget", "--spider", "--quiet", "http://127.0.0.1:8080/"]
       interval: 15s
       timeout: 5s
       retries: 3