fix: respect launch frontend port

[myeolinmalchi]

Summary

• Parse the port from the configured Frontend URL during launch setup
• Pass that port to pnpm dev / pnpm start so Next listens on the selected port
• Propagate PORT to background self-hosted starts and tighten URL validation

Verification

• pnpm format:check
• pnpm lint
• cd frontend && pnpm exec tsc --noEmit --pretty false
• cd frontend && pnpm dev -p 3199 (confirmed Next starts on http://localhost:3199)
• pnpm exec tsx scripts/setup.ts (verified setup prompt starts)