Refine installer step 2/4-1 and bind server to 127.0.0.1 by default

Step 2 (싱글/멀티): replace auto-advance with a card selection plus a
"다음" button so the user can review their choice before moving on.

Step 4-1 (모드 플랫폼): replace the "설치 / 건너뛰기" buttons with two
cards — "권장 플랫폼 설치" and "기본 마인크래프트로 설치". Selection
only records intent; the actual platform install fires in 4-2 along
with mods/resourcepacks (already handled by installer:client install).

Server: default HOST to 127.0.0.1 instead of 0.0.0.0 so the startup
log prints a Ctrl+클릭으로 바로 열 수 있는 URL. Set HOST=0.0.0.0
externally when public exposure is needed.

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

src/server/app.ts

@@ -6,7 +6,9 @@ import { indexRouter } from './routes/index'
 import { opRouter } from './routes/op'
 
 const PORT = Number(process.env.PORT ?? 3000)
-const HOST = process.env.HOST ?? '0.0.0.0'
+// 터미널에서 Ctrl+클릭으로 바로 열 수 있도록 기본값은 127.0.0.1.
+// 외부 노출이 필요할 때만 HOST=0.0.0.0 환경변수로 덮어씀.
+const HOST = process.env.HOST ?? '127.0.0.1'
 
 const app = express()