fix: restore sing-box after daemon restart

[structName]

Summary

• persist the user's desired sing-box running state when the service starts
• restore sing-box automatically when sbm restarts after power loss or daemon restart
• add a lightweight monitor that restarts sing-box if it exits unexpectedly while desired-running is set
• clear desired-running state on manual stop so stopped services stay stopped

Tests

• npm --prefix web run build
• GOCACHE=/tmp/sbm-go-cache GOMODCACHE=/tmp/sbm-go-mod-cache go test ./internal/daemon
• GOCACHE=/tmp/sbm-go-cache GOMODCACHE=/tmp/sbm-go-mod-cache go test ./...