fix(app-server-daemon): gate idle timeout constant on Unix

[andrei-hasna]

Summary

• compile the per-session idle shutdown constant only on Unix, matching its sole consumer
• preserve the existing 30-second Unix daemon timeout and avoid warning suppression
• clear the Windows -D dead-code failure seen in run 30381231457 jobs 90349313872 and 90349314146

Verification

• just check-fast -p codex-app-server-daemon
• just fix -p codex-app-server-daemon
• just fmt
• staged and origin/main..HEAD Gitleaks scans
• exact-head Blacksmith Testbox: https://github.com/hasna/codewith/actions/runs/30386894590

Native Windows GitHub checks are required before landing.

---

^{Need help on this PR? Tag @codesmith-bot with what you need. Autofix is disabled.}