env: commit .env.build with production SITE_BASE_URL

Track `.env.build` in version control so the production site domain
(`https://mc.tkrmagid.kr`) is baked into every portable exe build by
default. `.env` (server/dev secrets) stays gitignored.

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

.env.build

@@ -0,0 +1,4 @@
+# 빌드용 환경변수 — `npm run dist:win` / `npm run dist:win:rp` 로 패키징될 때
+# 설치기 exe 의 `resources/.env.build` 로 함께 배포되어 런타임에 로드됨.
+# 서버 운영용(PORT/HOST/SESSION_SECRET) 값은 여기 두지 말고 `.env` 에.
+SITE_BASE_URL=https://mc.tkrmagid.kr

.gitignore

@@ -7,4 +7,3 @@ conversations/
 .env
 .env.local
 .env.*.local
-.env.build