fix: use systemd for power actions

[alvins82]

What changed

• Replace legacy init 0/init 6 invocations with systemctl --no-block poweroff/reboot.
• Propagate power-command failures as HTTP 500 responses instead of returning false success.
• Add a regression test for error propagation.

Why

On systemd 259, /sbin/init 0 exits with Excess arguments. without powering off. The previous route ignored that error, so the UI reported success and later reloaded the page.

Validation

• Linux-targeted compilation succeeded for service and route/v1 with GOOS=linux GOARCH=amd64 CGO_ENABLED=0.
• Native macOS test execution is not available because the package uses Linux-only mount APIs.