fix: Fixed cross-platform issues

[gallayl]

🪟 Cross-platform fixes for Windows hosts

Two production fixes and a spec-path sweep so the service and its test suite run on Windows.

🐛 Fixes

• Yarn prerequisite check — execFile('yarn', …) fails on Windows because Yarn ships as yarn.cmd / yarn.ps1 shims that execFile does not resolve via PATHEXT. On win32 the call is now routed through cmd.exe /c yarn --version. POSIX still uses execFile directly.
• Graceful shutdown — ProcessRunner.killProcessGroup always passed /F to taskkill, collapsing the SIGTERM → wait → SIGKILL escalation in ServiceLifecycleManager into an immediate force-kill. The Windows branch now reserves /F for SIGKILL and uses plain taskkill /T for graceful signals.

🧪 Tests

• Added Windows-specific killProcessGroup specs asserting taskkill runs without /F for SIGTERM and with /F for SIGKILL.
• Replaced hard-coded /tmp/... literals with join(tmpdir(), '...') across 24 spec files (service, frontend, e2e) so the suite is portable to Windows. Two /tmp references in process-runner.spec.ts are kept intentionally — they pair with explicit process.platform = 'linux' mocks.ross-file link convention (CODE_STYLE.mdc).