fix(frontend): use relative API URL so browser works from any host

NEXT_PUBLIC_API_URL was hardcoded to http://localhost:8083/api which
gets baked into webpack bundles at build time. NEXT_PUBLIC_* vars
run in the browser, so "localhost" resolves to the *user's own PC*,
not the office host running the container — connect refused for
anyone hitting the site over Tailscale or LAN.

Use a relative URL (/api) and let next dev's rewrites proxy to
SERVER_API_URL (invyone-backend-spring:8081 inside invyone-net).
This way the request always lands on whatever host the browser is
already talking to (localhost / 100.126.230.80 / public domain),
and the Next dev server forwards it to the backend container.

Verified: POST /api/auth/login from browser path returns the spring
controller's 400 response, confirming the full
browser → frontend → backend chain works.

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

docker/dev/docker-compose.invyone.yml

@@ -61,9 +61,10 @@ services:
       - "9772:3000"
     environment:
       NODE_ENV: development
-      # 브라우저(클라이언트)에서 API 호출용 — 호스트에서 보이는 포트
-      NEXT_PUBLIC_API_URL: http://localhost:8083/api
-      # 서버사이드(SSR) 호출용 — 컨테이너 네트워크 내부 이름
+      # 브라우저는 상대경로로 호출 → next dev server가 rewrites 로 컨테이너 내부 backend 로 프록시
+      # (절대 URL 박으면 호스트/IP 가 머신마다 달라서 다른 머신에서 접속 시 깨짐)
+      NEXT_PUBLIC_API_URL: /api
+      # 서버사이드(SSR/rewrites)에서 backend 호출용 — 컨테이너 네트워크 내부 이름
       SERVER_API_URL: http://invyone-backend-spring:8081
       NODE_OPTIONS: --max-old-space-size=8192
       NEXT_TELEMETRY_DISABLED: "1"